Let me shoot a recent one out there (since this is a never ending thread :P ).

Getting ready to run E-Town. The day before I pulled the tranny and put in my LSD. Got the tranny back in and all set to fire up that evening. Tried to hook up the battery but it kept sparking back at me. I must have crossed or split some wires somewhere. Spent the next 2 hours search and tracing and finding nothing (But I did tear my car apart). Said F**K it and put everything back together and loaded up the car on the trailer anyway (taking a shot that I would fix it in the parking lot of the hotel). When I got up in the morning at the hotel, I just popped the hood and looked into the engine bay and thought about how the battery was acting like it reversed polarity........Mother F****r!

Yup, the battery I was using was a smaller battery from an Integra. I originally had to install the battery backwards so the poles were in the correct spots. I had hoooked the battery up backwards! :wallbash . Hence the reverse polarity, duh! Gee, she fired right up when I fixed that (and changed out the alternator fuse).

MB