I was able to find a table in your mydb that was somehow in a locked state, i.e. there was a race condition in which one process was trying to drop the table while another accessed it, and since the mydb tables are kept in a separate database the conflict was between the two databases. I removed the drop request and it appeared to clear the problem, but I now also see similar tables and activity in your account so I'm unsure whether there is still a problem. I do not think this is related to the size of the table and can't seem to reproduce the problem either.
If the problem returns please post back with more information about what you were doing and the specific table name. Hope this helps.