Okay, I have one...
I am kind of a detail freak and any collector car I have owned, I have spent untold hours detailing the car -- especially the engine bay. I have an old BMW M3 that is no exception. One time I had just finished changing the oil, when someone stopped by and interupted me. After he left, I closed up the hood and took off for a long, spirited drive. The car wasn't running well and wanted to stall at each stoplight/stop sign. I kept driving and tried revving it up higher and higher between shifts as the car warmed up to get it to run right. Eventually the oil light came on and my heart about stopped.

All I could think about was the $8,000 it takes to rebuild one of these engines.
I pulled over, shut her down and opened up the hood. Much to my dismay, the entire engine bay was covered with engine oil. Like an idiot, I had fogotten to put the oil filler cap back on.

I must have laid it down on the inner fender or something because I never did find it. It took me a couple of full weekends to get the engine bay back to the sparkling condition it was in before the incident. Fortunately, it did not result in anything more serious.
John
'82 Fiat Spider Turbo
'56 Abarth 750 GT Corsa MM
'59 Lancia Appia GTE Zagato
'62 Lancia Flaminia 2.5 3C Convertible
'68 Lancia Fulvia Sport Zagato
'70 Moretti Sportiva S2
'12 Abarth 500
'59 MV Agusta 250 Raid
Pictures of my baby!