As in bug #1441. mysql_pconnect().

To replicate:

1. use mysql_pconnect() to open a connection to a mysql database.
2. use the connection to run a query.
3. shut down the database.
4. try to use the connection; notice that it's bad.
5. ask for another connection to the mysql database; notice that it doesn't work (not surprisingly)
6. restart the mysql database.
7. use mysql_pconnect() to open a connection to a mysql database.
8. notice that the connection does not work, though the database is now available.

It seems the problem is in the connection pool. Bad connections should be thrown away so they can be used by good ones.