Just don’t. It never, ever, ever goes well. Even if you have all good parts — and you probably don’t, because you’re secretly stealing them out of graves because you’re committing crimes against man and God — any life you create is almost certainly going to be a hideous, nigh-mindless monster, so wracked with agony from being forced back to life that it will lash out at any living being in its path. The other possibility is that it will be a thoughtful, intelligent monster, so wracked with misery at its nightmarish existence it will dedicate its unnatural life to making you miserable, if not outright murdering you. Also, if you somehow come to terms with your creation, don’t try to create life even if the first life you’ve created asks you to create more. Sure, you think he knows what he’s talking about, because he’s a horrible abomination and you think he’d know the consequences. He doesn't. This still won’t work out. You’re just compounding the problem. Don’t do it.
2) Don’t set your inter-dimensional portal machine on a frozen lake.
Frankly, you shouldn’t set anything on a frozen lake, whether you’re a mad scientist or not. You’re just asking for trouble. But if you’ve got a large machine that allows you to travel between parallel universe — one that uses an intense amount of energy, and, you know, probably emits a fair amount of heat — you especially shouldn’t put it on ice. Because, as Fringe’s Walter Bishop found out when he tried to return to our universe with his parallel universe son Peter, the ice cracks, and then you both fall in an ice cold lake. And then in some timelines Peter drowns, ruining the whole point of going over there in the first place.
3) Don’t buy your materials from terrorists.
You’d think this would be a no-brainer, but many mad scientists are more interested in getting their resources — especially things you can’t just pop down to the Wal-Mart for, like plutonium — than where they come from. There’s no real win here; if you pay them for the stuff then you’re funding terrorism, if you don’t pay them then they get angry, and terrorists aren’t really known for willingness to forgive debts. Of course, you can do what Back to the Future’s Doc Brown did, and promise you’ll build some terrorists a bomb, receive the plutonium, and then use it for your own experiments. But that just seems a good way to get the terrorists and the U.S. government extra mad at you.
4) Don’t put your fiancée’s head in a pan.
Frankly, you shouldn’t be putting your fiancée’s head in a pan anyways, even if you get in a terrible car wreck that decapitates her and you have a lab at home with a special serum you’ve invented that keeps body parts alive, like Dr. Bill Cortner happens to have in The Brain That Wouldn’t Die. I’m not saying you shouldn’t try to save your fiancee’s head, just put her in something nicer than a baking pan, for god’s sake. Like an aquarium, or even a fancy ceramic bowl. She’s really had a bad enough day, don’t you think?
5) Don’t use the fact that your fiancée’s head is in a pan to go to strip clubs to ogle “prospective bodies.”
Look, I understand. Your fiancée is a head in a pan, and she needs a new body. The inclination to use the opportunity to “upgrade” her measurements has to be pretty strong. But two things: 1) she’s probably not going to appreciate the “research” you’re doing, especially if it takes more than a couple days, and 2) she’s definitely not going to appreciate it if she used to be a B-cup and you suddenly outfit her with a pair of DDs. There’s no win here. Especially if you leave your fiancee’s head in the room with an angry mutant and she forms a psychic link with it while your putting singles down Sapphire’s g-string with your teeth. Because she’s going to have that mutant beat you to death, possibly with her own pan.
6) Don’t listen to aliens that say they want to help.
Because they’re lying. They don’t want to help; they simply want to take over the planet and/or eat us, like in V. Aliens never want to help us, and why should they? We’re insane, destructive monkey creatures — no alien with a lick of sense would want to give us advanced technology or let us roam through the galaxy, nor should they. And if you’re a scientist who discovers an alien that wants to help humanity by taking over the world — e.g., It Conquered the World, in which the alien offers to end all war by basically mind-controlling everybody on the planet. Because while there may no longer be war, it's because we’d all be mindless slaves to a giant cucumber with a face. Not worth it, bro.
7) Don’t turn yourself into a goofy animal.
Hey, if you’re determined to turn yourself into a terrifying human-animal hybrid, I’m not going to stop you. But you need to make sure the animal is something that’s not going to make you look or sound like an idiot when your victims see you running at them in the dead of night. Cool animals include wolves, lions and sharks. Dumb animals include penguins, sloths, and platypuses. Also, don’t be like Dr. Z of Blood Waters of Dr. Z, and pick something waaaaay too specific, like the walking catfish. Maybe being a human/walking catfish hybrid is super-awesome and you have tons of cool new abilities (although Blood Waters of Dr. Z gives no evidence this is true). But walking catfishes don’t have a particularly fearsome reputation, meaning when people hear there’s a walking catfish-man terrorizing the nearby lakes and rivers, they’ll be more confused and bemused than afraid. Why make things harder on yourself?
8) Don’t choose the corpses of murderers for your experiments.
You are performing an experiment. This means you don’t know what will happen. So, if you’re performing an experiment on a corpse, there is always a chance that what you’re doing could result in the corpse returning to life, possibly with super-powers. Sure, maybe you’re just seeing what will happen when you feed an irradiated Mentos to a dead person and it seems super-unlikely that he’ll spring back to life, but you don’t know for sure. But if they do happen to come back from the dead, they are going to murder you and everyone else in your lab, because they are murderers, and that’s what they do. So rather than using the bodies of killers and other convicted criminals for your experiments, why not use the bodies of nice old ladies? That way, even if they happen to come back to life and are twisted and evil and are indestructible or something, at least you can probably outrun them.
9) Don’t get bent out of shape over a small facial scar.
Human bodies are not indestructible. We get hurt, we get ill, we get scarred; it’s just a fact of life. So if you get a small nick on your cheek, donning a giant suit of armor and vowing to kill the other scientist partially responsible for said scar is overreacting, to say the least. I mean, just put a goddamn band-aid on, Dr. Doom. Or, if it really upsets you so much, go to a plastic surgeon. Seriously, I feel like the scar was the excuse to put on the armor and become a supervillain, not the cause.
10) Don’t emotionally abuse your son if you’re going to need him later to pilot a giant robot.
So the apocalypse is here, God is sending giant monster angels to destroy what’s left of humanity, and the only person that can stop them is your 14-year-old kid that you’ve shat on for the last 10 or so years of his life. Is he going to want to pilot the giant robot for you? Of course not. Is he going to be a giant crybaby? Of course he is. Is the whole thing going to be a massive ordeal? Definitely, and the fact that robot contains his dead mother’s soul isn’t going to make things any easier. If your plan for saving the world (or helping humanity transcend itself, or whatever the hell Gendo Ikari in Evangelion is trying to accomplish) is dependent on your kid‘s emotional well-being, then at least take him to Applebee’s on his birthday while he’s growing up.
11) Don’t lie to yourself about your madness.
Look, you may think half the fun of being a mad scientist is getting revenge on those who’ve wronged you. And it’s true; if your fellow scientists branded you a lunatic, then by all rights, you’re allowed to make an army of atomic supermen to kill them and take over the world. But don’t pretend that this, somehow, makes you sane. Killing your colleagues via atomic superman is the very definition of insanity, and so all you’re doing is proving them correct. Poor Dr. Eric Vornoff in Ed Wood’s Bride of the Monster is guilty of just that. Remember, be a mad scientist, not a sad scientist.