I had a nasty MySQL problem arise the other day. I added two new columns to table 'Promotions' (13 existing columns, ~750 records) using the PHPMyAdmin interface. This query ("ALTER TABLE.....") never fully executed and locked the table, so that each subsequent query to 'Promotions' would hang and queue up. It took me about an hour to realize this was happening, and the problem was resolved by me rebooting my database.
Are there standard precautions that need to be taken and known issues that arise from altering database schemas? I have never seen this kind of locking happen before.