Longshoot did a very good research in his answer but he should also include key no. #0000.
The story behind is quite unusual.
Originally keys #0000 were made uniquely for GMs. They were able to open literally every doors in Tibia! Even the peoples' house doors.
As you might know, with great power comes great responsibility. And many people can't resist temptation of abusing it. That's exactly what happened in this case. Somehow this key landed in private hands. Imagine having a key that could open doors of Kazordoon Treasure Room doors with Dragon Scale Legs, Golden Helmet, Crown and Silver Necklace behind..
After Cipsoft found out about that situation, they immidiately removed keys' #0000 usage.