I've told this one before, but it's a new thread, so wth.
*sigh*
This is the first bike I've ever worked on, so I had loads of misplaced confidence before having the Fear scared into me.
I did the valves. Myself. Sorta.
I'll let that sink in for a second before sharing the full repercussions.
Ok. So. I think I set them all at TDC 1 instead of TDC 1 then TDC4 respectively. I know for sure I didn't torque the locknuts down enough. For the rest of my errs, I will never really know, but on a hot summer day, next to a friend's house, on the dry grass, I decide I'm going to use a squirt of ether instead of fuel to get the engine to start, because, toiling away in the sun, I've done pretty much everything else. To my (continuing) surprise and amazement, it turned over and coughed a little, but ran. I wanted to repeat this marvel of my own undertaking, so I squirt in more and more ether. This is about where the engine backfires and coughs still-burning ether into the airbox, thereby igniting my filter. OMFG. Thankfully the tank was off, and I was able to find the garden hose by the house.

The lasting effects of this valve adjustment were two bent valves and the ensuing top end job. Oh, and MX continually reminding me of how I called her while she was in the OR telling her:
" *static, static, the bike was on fire.... static, static... ok now."