I tried to run a data migration query on a live database server, and after a minute, I could not connect to MySQL via the command line due to "Too many connections." I had to kill the query and restart MySQL. I won't try to run such a query on a live system during the day, again :)
This same query took less than two minutes on very low-traffic copy of the production server, finishing fine.
Is it likely that the traffic to the server would have made this query stall?