Firstly you would expect that you can't break blocks while blocking/charging a bow/eating (see bug report MC-2710). But ignoring that, you would expect that when you release the Left Mouse Button, the breaking animation would go away, but under certain conditions, the breaking animation will not go away.

to reproduce, follow these steps:
1. hold a sword/food/bow
2. hold both Left Mouse Button and Right Mouse Button
3. before the block breaks, release the LMB and then hold it again
4. before the block breaks, release the LMB and then release the RMB
5. the breaking animation will now be frozen on the block, although it will reset if you try to mine it again (interestingly, it will not reset if you are holding RMB when you try to mine it again)