I once walked into a small store to buy cigarettes. The guy at the counter was counting €20 bills. I asked for some cigarettes (€5) and paid with €10. When he gave me my €5 change I was confused because I thought I gave €20. I told him I gave €20 and he looked at me, thought for a moment, and then gave me the €10 and said sorry. I went home, took out the €15 and only then realized what happened.

Our minds are feeble little things.