Days hold a lot of meaning in Thailand (Thai people will often ask farang what day they were born on and be blown away when we don't know) and there's a whole host of superstitions surrounding each one. The most obvious is "no cutting on Wednesday" - this includes trees, fingernails and hair, and is so widely believed that pretty much every salon and barbershop is closed on Wednesday. Others include no withdrawing money on Thursday, no funerals on Friday and no beginning construction on Saturday.
According to conventional wisdom, eating durian and drinking alcohol WILL kill you. There are tons of articles online about people who combined the two and promptly died in their beds. This probably comes from the belief that durian and alcohol are both "hot" foods and so will result in the body overheating. A more modern explanation is that durian contains a lot of sulfur, which speeds up alcohol's absorption and will get you really drunk really fast. As far as we can tell there's no truth to any of that.
It's a common belief throughout Thailand, Laos and Cambodia that green snakes and tokay geckos (the big blue ones) have a sort of symbiotic relationship. Supposedly, tokay geckos' livers can grow so large that they become painful. When this happens they let out a specific noise to attract green snakes, which then crawl in through their mouths, eat part of the liver and everyone leaves happy.
Obviously this presents a few biological and anatomical problems. But wait! There are videos of this actually happening! Googling it will also give you a few intriguing images. What's actually going on is that the green snakes are often not big enough to actually kill and eat the geckos, only immobilize them. Rather than leave empty handed (or, you know, whatever snakes have) they'll stick their head into the gecko's mouth and eat whatever is left over from its last meal. Which is almost as weird as eating the liver.
There are a lot of superstitions about pointing at things (and indeed, it's just rude in general), but the ones surrounding rainbows are definitely the weirdest. Pointing at one will result in your finger falling off, and we've heard from some sources that the only way to counteract this is to stick it straight up your butt.
At Thai campgrounds you'll sometimes see a bunch of limes on sticks surrounding the tents. They're put up with the intention of repelling snakes, which you definitely don't want in your tent. This is completely false, but interestingly this belief also exists in the West. It's more often attributed to lime the chemical, but that's apparently not true either.
Women's menstrual blood having unholy powers is nothing new, but some guys in Thailand take it very seriously. Anything a woman wears under her waist - underwear, pants, sarongs - has the power to instantly sap a man of his virility. Ladies, don't ever ask an insecure dude to do your laundry.