“It’s dented” “it’s also free”

I work at a paint store and we can give away some empty gallons to customers if they need them. A woman comes in near closing and asks for two empty cans. Normally those are for paying customers only who are already buying paint but I was feeling generous. I inform her I have two left and she says “okay get them”. I bring them to her and she points at one and doesn’t say anything. I turn it around and there is a small dent, nothing too big that would mess up the gallon at all and I said “oh don’t worry, that’s a small dent and you’ll be able to put a whole gallon in there still”. She looks at me and goes “but it’s dented. I don’t want that one” and I said “well it’s free, so...”. We kinda stared at each other for a minute and she huffed and took it and stormed out. I sat there and laughed and later found some other gallons in the back that weren’t dented. Whoops