What have you recently done that was a huge mistake?
-snip-
Oh wow, you've got to win some kind of award for that.

What did your friend say?!
This isn't recent, but it was a pretty stupid thing for me to do.
A year or two ago I had to have loads of blood tests, and I used to pass out nearly every time. One of these times, I didn't feel so bad and got up quickly to leave. In the corridor I started to feel really dizzy, but it wasn't far to the car park and I thought I could make it out into the fresh air before I fell unconscious. I started running (well, staggering) and everything started going black just as I made it back through the waiting room. Apparently I passed out in front of everyone, cracked my head open on the metal door handle and fell down. I woke up with the horrified faces of two receptionists, a nurse and an old lady looming over me.
I felt like such an idiot. Clearly I was never going to make it to the fresh air. As soon as it went black round the edges, I should have lay down where I was and saved myself the humiliation.