Failed to add the field to the table/feature class

I was having this problem when trying to add a field to an attribute table and couldn't think of a reason for why it would be happening. I checked the ESRI Forums and found someone else that had the same problem and was able to resolve it by changing the write/permissions in the folder where her shapefile was stored. I already had write permissions so it had to be something else. I figured out the problem, I had the file opened in another location on my computer. If you have this problem, make sure that it is only open in ArcGIS Desktop.

-Alex