See that? That is the Thunderbird dock icon on Mac OS X. It shows the number of unread emails you have.
What looks like “1487!” is actually “14875” with part of the “5” cut off. I have discovered the dock icon can’t display 5-digit numbers correctly! A bug!
Hm. Perhaps this is the rare case where it is, in fact, the user’s fault for having 14875 unread emails. What should Thunderbird do when you get to five digits? At this point the exact number is not particularly informative. Maybe when you get to 5 digits, it should just show an infinity symbol. Or a sign that says “You’re screwed”. Or a link to inboxzero.