Fully prepared to be told I am being unreasonable for this...
So a while back my friend asked me if I wanted to go to a concert for a band we both like. We'd been to see this band together before and this tour was for a new album so I definitely wanted to go see them again. She said she would buy the tickets and I can send her the money at a later date.
The day tickets go on sale she messaged to say she got us the tickets, but thinks she messed up and bought more expensive ones by mistake. I asked how much and she said £40 each which I actually didn't think was too bad and not much more than what we paid last time. I offer to pay and she said to leave it until closer to the time.
A few months later she asks if I can send the money across, I asked her how much it was exactly and she said £50... I felt a bit uncomfortable saying she told me it was cheaper so just made a joke like "wow who do they think they are charging that much haha" and sent her the full £50.
Concert day arrives and she has the tickets in the envelope they came in, and I can see through the window of the envelope that the tickets actually have £70 printed on them... I didn't mention it, then she kept making references to the tickets and making sure the price was always on clear view to me, even leaving them face up on the table when we went for a meal before the concert.
I know it sounds horrible but I was so fed up by this point with the price increasing from £40, then £50, then £70 that I just never brought it up. If she had just said at the beginning that she messed up and bought £70 tickets I wouldn't have minded. It was more the principle that I'd already paid the price she told me and wouldn't just ask me outright for the rest.