Bosses that come back after you think you've defeated them. It used to get me all the time, I'm like YES, forget YES I FINALLY BEAT HIM, SUCK IT friend, YOU ARE DEAD, hah-oooosht, and then they come back twice as strong. Now I'm always skeptical about these things. In portal I thought she was going to break out in laughter and tell you that her destruction was a joke like she did with the gas
"nuro toxins, cough cough, so deadly-HAHAHAHAHA"
...how do you spell nuro neuro nero?