I have one, as a single mum about 6 years ago, I had cut back for a few weeks to pay a fortune in babysitting fees to go to a friend's birthday karaoke in London. I took the train and had a return ticket.
Afterwards, my friend and her partner, her sister and sister's partner saw me walking to the train station and said I should share their taxi. I said I had a train ticket already (obviously not outright wanting to state that I couldn't pay for a taxi). The karaoke place was in zone 1 and we lived in zone 6, so a taxi would be at least 50 quid. Anyway, they insisted I go with them, and off we went.
My friend and her partner were being dropped off on the way in zone 4, and the sister and her partner lived in my town, nearly a mile from me. We drop friend off, and because it was her birthday, the sister's bf says 'oh, don't worry, we'll cover your taxi as it's your bday'. Off she goes, happy as Larry and then I started feeling an anxious pit in my stomach as the ramifications were starting to hit me.
We are about 5 mins away, when we get a call from my friend - 'oh our babysitter needs to be dropped off'. Off we go back in the taxi to get the babysitter who lives close to us as well. So we're now adding another tenner at least. I had my back to the meter so couldn't be absolutely sure.
Cutting a long story short(er) the taxi ended up being 75, and when we get to their stop (before my place) the boyfriend shoves 40 at me and cheerfully says goodnight. I was absolutely floored and too cowardly to say anything. The sister did look a bit shamefaced.
I had to get the taxi driver to drive to the nearest cash point which was luckily about 300m away, draw out 40 quid as I felt I had to give a tip, and walk another mile home.
I never told my friend, but I could never look at her sister in the same light again, and they broke up shortly afterwards. That evening ended up costing me 110 quid in train ticket, taxi, 1 drink plus babysitting. (And I sang like shit :))