Error on posting after leaving page with the "loading" bar going (5+ seconds)

The title of the thread post, but the thread view will say this to the user:
Code:

A server error occurred. Please try again later.
And this to an admin:
Code:

XF\Db\Exception: MySQL fetch error [2014]: Commands out of sync; you can't run this command now in src/XF/Db/AbstractStatement.php at line 230
[LIST=1]
[*]XF\Db\AbstractStatement->getException() in src/XF/Db/Mysqli/Statement.php at line 207
[*]XF\Db\Mysqli\Statement->getException() in src/XF/Db/Mysqli/Statement.php at line 142...

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