You start a long-running command that you know will take several minutes to finsh. So in the mean time you switch away to check email/facebook/whatever. Hours later you realize that the command you started has been finished for ages but you forgot to switch back.
This was literally happening to me almost every day, often several times a day. I could probably have solved world hunger with all that lost productivity.
This simple script might just be the solution to your problem. When you know a command you're about to run is going to take a while, prefix it with 'remind' and get notified when it terminates. Better yet, a handy 'Go' button will take you right back to the window where you ran the command.
adam@brigantine:/usr/src/linux$ remind make