MySQL ALTER and DROP trouble

Hello!
Well, actually the problem consists of the following: I have a rather complicated php code that connects to the MySQL creates a database, creates several tables in it and performs some initial inserts. After that it provides some means of altering tables' structure (via ALTER TABLE and DROP). So, the problem comes when I try to alter tables after a SELECT statement was executed. MySQL issues error that it can't change the table (errcode 13).
And if I alter tables right after the creation process everything works like a charm. Where is the problem??
And the second question is: where can I find a decent documentation on MySQL errors?

Thanks in advance, any feedback will be greatly appreciated.

Hmmm..I would try to issue a FLUSH TABLES command right before the ALTER. As fdor finding error codes, I normally just do a google search for them...you can also search the mysql docs at :

http://mysql.com/doc

Thanks again! The FLUSH helped completely.