Guess I'll chime in; I've totally done this too. Checked the oil during a fuel stop, topped it off and the closed the hood and took off. A little way down the road smoke, and a nasty smell, started billowing out from under the hood. My heart totally sank and I got a very sick feeling in my stomach (and I needed to change my shorts) because I thought that the engine was toast. Pulled over into a parking lot, popped the hood and saw oil EVERYWHERE in the engine compartment. After a slight panic I realized that the oil cap was off, and luckily still sitting where I had left it (hood must have kept it in place). Topped the oil off again, replaced the cap, and went on my way.

I don't think the engine compartment was completely cleaned until the engine was replaced (not due to oil loss).