Damn I miss homebrewing.
The time I skunked it was when I was trying to make a batch of some sort of ale with maple syrup in it. It served the role of some of the sugars for fermentation. I just opened the carboy, opened the jar of syrup, and poured it in.
Got a little lax with sanitation - didn't wash my hands, probably should have boiled the syrup first, etc.
A weeks later I snuck a taste at bottling time and it tasted like gauze. It wasn't going to get any better in the bottles, so I just dumped it.