When creating the user account "17iwang" on the English Wikipedia on behalf of a student, I received this error message:
Last attempted database query: (SQL query hidden)
MySQL error: 1213: Deadlock found when trying to get lock; try restarting transaction (10.0.6.48)
I tried to repeat the create operation, but was told:
"The requested username is already taken in the unified login system. Please choose a different name."
I visited the user's user page, but it stated:
"User account "17iwang" is not registered."
Eventually I worked around this issue by logging in to the English Wikipedia as the student, using the password I supplied during the initial create attempt (the one that produced the deadlock).
It appears to be rare and based on concurrent transactions - I've never seen it before or since, despite creating many accounts.