I have a couple of websites using MySQL. Table metadata gets corrupted from time to time, especially on very active tables, and as a result the table can no longer be accessed. If it's a critical table, like for example the one containing forum posts, the site will be down. If it's less critical, like a tracking or statistics table, you might just see errors, and the site could slow down. The corrupted table can be fixed in a few seconds, but to do that you need access to a MySQL client.