Can I just apologise again for attempting to steal your seat!
To explain - I was travelling home on the District Line last night, and was standing in between the seats. When we pulled into a station, a woman sitting in front of me got up. I assumed she was getting off the train, so I immediately sat down in her seat, but actually she was getting up to give her seat to a pregnant lady. When I realised after about 2 seconds, I was so embarrassed, I jumped up, bright red in the face, and apologised. As penance I refused to sit down for the rest of the journey, even though there were lots of seats!
I always, always, always give up my seat for pregnant women and old people, and I was absolutely mortified to have done this. Thank goodness I have the Mumsnet confessional!