The 3 most awful guitar mistakes:

1. Placing your guitar to the rear of your car preparatory to loading it, getting involved in a short conversation with someone, getting in your car, and backing it out over the guitar. (I almost did that one time, and I know someone who HAS done it, totally destroying the instrument.)

2. Placing your guitar on the sidewalk after a late night downtown gig, hailing a cab, getting in the cab without the guitar, and driving away! (Know someone who did that too...)

3. Tuning the strings on your guitar up way too high, causing the bridge to be ripped off the instrument! (Yes, people have done that one too...)

And after that....

4. Lending your guitar to some moron who puts pick scratches all over it in just one 3 minute song!

5. Taking both hands off the guitar while wearing a strap and the strap lets go. (someone already mentioned that one above)