It's definitely not supposed to do that but could you explain more clearly what was happening? A few good places to start:
1) When did you first notice this behavior? As soon as you got the item? At some point later?
2) Did you try refreshing your client? You probably had another invisible item in the second slot, and I'd be interested to know what it was.
3) Did you get any error messages during this time? If so, did you submit them with notes, and if not do you remember what the error message said or about what time it happened?