OK; Here is my story. I put an  Ohlsson 60 on the test stand for a couple of runs the other day. I have this little strap of brass with a wire soldered to it for the ground back to my ignition box. The strap is supposed to go under a lug on the engine when I mount the engine. Well! I did not quite get it there and it was just touching the engine lug. I do use a transister in my ign., so even with it just touching it would start, but ran really rough with a very strange miss. This engine normally runs very well and I was really confused. I cleaned the plug and points, checked the filter, and could find nothing. I was about to give up and take it off the engine stand and try another engine when I happened to notice the ground strap kind of floating beside the engine lug. I put the strap where it belongs and all was well again. Hopefully I won't make that mistake again for another 6 months.

I am afraid I have had this all my life. My kids still laugh and won't let me forget the time I was in a hurry to get the carb back on my lawn mower to get the mowing done. I took it off to replace a leaking mounting gasket. I did not have one so made one and installed the carb. The mower would not start. After trying to start it for about 15 minutes I realized I had forgotten to cut the hole in the middle. 

How about fighting multiple short runs after a hard landing with a clunk tank?  It won't fly long with the clunk in the inside front position.

I most RECENT, uh, intellectually challenged moment came when I set up my first electric, then could not get it to start.  Talk about helpless.  Turned out to be a broken wire in the arming switch....  Like Jim K pointed out, electrons don't like gap-o-sis in the wiring!

Ty,  thanks also for the copper tubing tip. I use smaller 3 oz. clunks mostly and the supplied silicone tubing won't quite flex enough for my liking so i use latex tubing for the pickup. I have had the "forward pickup" thing happen a couple of times, luckily only got short runs no crashes!!