Here's a nice little situation:

You're nearly master of both the fighter's guild and thieve's guild. You speak to the leader of where you're at right then, and he asks you to kill the other one. So, you go to the other guild, but before you kill the person, you talk to him and ask if he has anymore quests before he dies. His quest is to assassinate the leader of the other guild. NBow you have two rivals asking for the death of the other... who do you kill?

The answer is the master of the fighter's guild- Sjoring Hard Heart. Kill him and you'll advnace from the champion to the master. Report back to Gentleman Jim Stacey, and he'll promote you to master of the Thieve's Guild.