I could probably write a book about the annoyance of TFL.
I'm sure I wouldn't mind TFL so much if it wasn't so bloody expensive and getting more expensive. When I first arrived, bus fares were £1 but then it went up to £1.20 and now I think it's £1.30. It's up 30% and I have only been here for less than two years. I've met people who swear it was once 90p and below... Also a one day travel card for zones 1-2 used to be £5.70-ish, now it's almost £7. I was at Heathrow recently and some Americans were asking me which fare they should buy. I remember going with my usual advise to just buy a fare into the city then buy a zones 1-2. However, the one of the security guys corrected me and told me that it was cheaper to buy a 1-6 day pass (for virtually all of London) then to pay for a 1-2 with two fares

Granted, they were only there for a day and weren't gonna bother with queuing for an oyster card (which probably wouldn't have saved them much anyway!) I might just start telling people don't bother with the one day card if you're not gonna travel more than 3 times...
I think my cards are messed up. Recently I lost a £5 top-up. I put five pounds into the machine, the top up went along normally, tapped in and out, etc. I got to the bus stop and learned while trying to tap in that suddenly my £5 was gone! Thankfully the driver waved me through...
The other day, I let a friend borrow a regular oyster card (I keep a regular and a student) since she left hers behind in a hurry. We topped up £5 each at the station. The regular was minus 10p while my student had 20p left, so respectively we had £4.90 and £5.20. We went the same exact route: Southeastern service to Victoria and changed for the tube to Vauxhall. When we got on the bus later that night, her card worked but mine claimed that there wasn't enough money!

I know for a fact that I tapped in and out at each station (I know if I hadn't they would have charged me over £4! and I checked my balance later and I still had 80p left so I know they didn't charge me for not tapping out). Grrrr you Tfl!