Sometimes while working out I fail so suddenly and spectacularly that I can't help but have a fit of laughter. This happened today while doing supersets of push-ups and seated rows. Plank position, down, and... whoops! Face meets floor. Laugh incoherently for a little while, then back to rows.
But sometimes, the laughter causes the failure. A few weeks ago, my trainer was spotting me during bench presses. Toward the end of my set, he was standing over me, prepared to take the bar if needed. I exhaled so forcefully through pursed lips that it actually made him squint and caused his hair to shoot straight up. I immediately started cracking up, and apologized (while still laughing). He was a good sport. Needless to say, that was the end of that set.
Gotta have fun with this stuff.