Here is a video showing what not to do when playing goalkeeper in soccer. Watch at this local rec game how this unfortunate soul became the butt of jokes amongst his fellow players.
Clearly the goal post should have been called for a foul for that vicious tackle.
To summarize, here are three things you don't want to happen to you as a goalkeeper.
1) Don't get nutmegged.
2) Don't get tackled by the goal post.
3) Don't be a laughingstock for your friends and rivals.