There's a woman in my department that I've thought for a while was being a bit 'funny' with me. Nothing too obvious, just a bit brusque and offhand. I only know her to say hello to and don't work with her too often so haven't given it much thought, and thought maybe I was just being a bit paranoid.
Anyway things came to a head today. Someone was doing a lunch run to a sandwich shop, so I gave my order, then went in my bag and realised I'd left my purse at home.
So, I went over and apologised and cancelled my order. She asked why and said not to be silly, she'd pay and I could give her the money back. So I thanked her, and she said she'd pop over tomorrow to get the cash.
Then the woman who I mentioned above, who happened to be sitting next to her, went 'hmmpph! You'll be lucky' 
to colleague.
So I asked her what that was supposed to mean and she said well i'd still not paid her money owed so I obviously couldn't be trusted.
It turns out that in August I put my name on a form that came around the department and sponsored her for a fiver for some charity walk she was doing. It took me a couple of minutes to remember, but I did...I pledged the money and that was the last I've heared. I just haven't thought of it since.
I did a 
face and asked why on Earth she hadn't just reminded me after she'd done the walk. She has my email address, I work in the same department, she sees me at least a couple of times a week and knows where I sit...it really wouldn't have taken much.
This is completely ur according to her...as I made the commitment, I should have ensured she was paid and it's not down to her to 'chase' me.
Aibu to think that if you ask people to sponsor you, you should also take responsibility for collecting money once the sponsored event has taken place?