If I am tired enough, I can basically sleep anywhere.
I have been known to fall asleep sitting in waiting room chairs, crowded trains, in between arm rests and seats of airport chairs, in bathrooms, but perhaps one of the most surprising is during medical procedures.
I had an MRI of my sacrum done last night at the hospital, and when it was over and they came to let me out, they said that they couldn't believe how perfectly still I had been for the entire test. Truth is, I was perfectly still (other than a couple of involuntary twitches) because I was fast asleep in the MRI machine. As loud as it is, I find something soothingly rhythmic about the noise... And I was exhausted. If I have to sit perfectly still for over half an hour, I would rather do it sleeping. :)