I believe you have Type Marked a wall with the number that has been delected or not used. You would have to go through all of the wall types you currently have that are not being used and check for a Type Mark parameter setting for the number in question. Two wall types cannot have the same Type Mark or Mark.
Mel Persin, AIA
AEC Technology Consultant
Technology to Visualize and Realize Solutions
Modeling for the Future/Drafting in the Present/Building on the Past