While fixing a bug preventing a character from putting something in a locker containing a bugged item, I discovered another bug introduced in February of 2014. This bug caused lockers to not include the weight of existing locker contents when calculating if a new item could be placed inside.
Since the bug was introduced some time ago, its quite possible you have been able to put more items into your locker than it should be able to contain. Because of the way the code works, you may need to remove almost everything from your locker (because the last item remaining inside -could- be taking up all the space). Please do not report this as a bug. :)