Boy do I have a few but I'll name the best ones.
First was finding that World Racers set 8898 for $0.99 at a thrift store. The box was battered but could still hold up on it's own and after opening the box the contents were still neatly organized. Pieces sealed in their bags, stickers in excellent condition, instruction booklet looking like it was hot off the printing press, everything was still crisp. I haven't built it as I'm debating whether to build it and use as a set or scrap for parts. Either way it was an excellent find.
The second one is when I was browsing a Target and found a Fire Car polybag set 30221 in bulk quantity. Seeing the firefighter caught my attention and gauged my interest in buying it but that's not the real find. I found a small number of LEGO Movie sets and this was back in December 2013, three days before New Years. I felt compelled and found the CMF bags. Carefully feeling them I didn't which to take so I took one and made my way to the cash register. Upon scanning, the cashier couldn't scan the CMF bag as it's data made it un-purchasable until the beginning of 2014. After seeing that I thought I might have to return it but instead the cashier put in my bag along with my original purchase saving me a few bucks. Needless to say it was a good night.