#1 They're smarter than you think. If you have any presumptions as to what they can/can't do, they will bust them all wide open. Do not be surprised if you wake up one morning to find them trying to crawl into the microwave—which, yes, they have somehow managed to pry open.
#2 They do not respond to their name. They only respond to VERY loud noises. So, if you really don't want it to settle down in the bathroom sink, frighten it with a very loud sound and make it scurry away.
#3 They hate being on their back. If you're transitioning from being a dog person to a cat person and are thinking, 'hey, dogs love having their tummies rubbed! What could go wrong?!' prepare to be mauled by a set of mini-claws. Being on your back is a servile position, and if there's one thing your cat does not like, it's servitude.
#4 You will, atleast once or twice in the early days, awaken to them standing over your head. Do not be alarmed. They are not plotting to kill you, they are simply bored and not sleepy and without things to do.
#5 They are also more stupid than you think. You might believe that, at some point, they'll get bored of the airplane baggage tag hanging from your carryall, but fear not. Your beloved creature will return to it on multiple occasions in the course of the same evening, their fascination rejuvenated, their thirst to battle that odd, rectangular alien force unquenched.
#6 Their memory resets every ten minutes. You might think that, over time, you have formed a bond with your cat and it loves you. But it looks at you every half an hour thoroughly confused as to who you are and why you're there and when you plan to feed them.
#7 If they get nasty, that just means they're hungry. So, if you see pure hatred in their eyes, just pour them some kibble and all will be dandy.
#8 You CAN leave them on their own for ages. Unlike dogs, who start to miss you in a couple of hours, cats couldn't really give less of a sh*t if you're around or not. Just leave them plenty of food. Proof of this fact: come home, and they'll show zero interest until you nab them while they're running around and force some cuddles on them.
#9 You can also have sex in front of them. You'd imagine that the movement would confuse them and you'd have angry, red scrawls across your bare behind before you know it. However, due to their earlier policy of not really giving a sh*t about you, they will leave your coitus un-interruptus.
#10 They don't just purr when they're happy. It might be thoroughly pissed off, and whilst you may be beaming because it's sitting around in your arms whirring away like a motorboat, beware—it could scratch your eyes right out of your sockets.
#11 They ONLY meow to talk to you. Turns out, it's a scientific fact that cats don't even meow at each other—they only use it to communicate with human beings.
#12 Don't bother mopping up its litter six times a day—they will only spill it again in about five minutes. They love their litter box—even more so than their bed. Or YOUR bed. Scoop it up for bedtime cuddles anyway.