I was finishing up five sets of skull crushers (behind the head) and I couldn't get the last rep up and forced it, tris were burned...and bam! Got hit on the crown of the head! good thing nobody saw it...
I went on to do 5 sets of dips...head hurts, but tris hurt worse, pride intact.
This is the last time I'll try 6 for 5 (six diff exercises for 5 sets).