When you run out of flags for the day, the flag dialog still allows you to go through the process of flagging up until you actually click the button. You drill down through the menus, find the reason you want, and press the button only to see something like this:
Obviously, you get a little number at the bottom telling you how many flags you have, but the button is still enabled (implying that you can still flag):
At the very least, shouldn't the button be disabled even after you choose a flag reason if you're out of flags for the day?
However, what would be even better is if it were indicated on the post itself that you are out of flags via the disabling of the flag button in the way the edit button is disabled on per-site metas for users without full editing privileges: