I'm surprised this issue seems to have no previous threads - at least I didn't find them. This happens to me at least once a week.
At different times CR XI pops up with "font still in use." It doesn't seem to harm anything (I'm not blocked from proceeding) but I wonder what's up.
I'm not 100% certain but I'm pretty sure it happens when I'm closing out of something - either closing a report, or perhaps closing an instance. (It's not usual for me to have a second instance of CR open, so I can do something while a long query runs.)