This one is going to be quite hard to replicate due to time restrictions and I'm not 100% when the character goes "away" but might help a select few in a big way.
At some point after quitting the game for a while all your characters will "disappear" and you'll have an empty realm select list.
Blizzard's recommendation is to look on the forums for the character names, click on that realm, make a new character, hit refresh, and lastly to delete some folder. DON'T DO THIS!
Now for the part that has to be confirmed and hard to replicate, is one of two things happens. Either your character/s is/are forgotten already and you can make 50 characters now, or some point in the future they are forgotten and you can make 50 characters.
Now the way it happened for me was one of these old forgotten characters (created shortly after BC) got flagged for a rename and suddenly appeared on my realm select list after I was already maxed on characters and put me at 51.
Because of how hard this is to replicate and my lack of preparation I haven't done any experimenting to figure out exactly how to replicate it. But if you know the name and realm of your old "forgotten" character you might be able to speed up the rename flagging step by requesting the name on another account to flag it.
Proof (and no risk of ban):
gmresponse.jpg