Supposing you had a MySQL table that stored user details, for simplicity lets call the table 'users' and you had 2 indexes:

1) The auto_increment primary key (users_id)
2) An index on the username (username)

If you were to run an update on a row for example:

UPDATE users SET first_name='tom' WHERE users_id=10 AND username='tomtank';

Naturally you would expect this query to run and perform the update. However, thats not what I discovered. What actually happened was, MySQL ran the query but did not perform the update and reported zero rows affected. However, as soon as I removed the second condition in the WHERE clause, the results were as expected.

Of course one may argue for the necessity of the second condition in the WHERE clause, but I did that simply out of vanity. :)

This is a strange one indeed, and might though I'm not 100% sure be attributed to my version of MySQL (5.1.33). Whatever the case is, I just thought I would share this experience in the hope that it  may save someone alot of time and digging.