mysqldump unusual error

A friend of mine called me today, asking for advice on a baffling error.
He has just finished upgrading a database server from 4.1.20 to 5.0.45but, when attempting a backup to all databases with mysqldump, he gets a discomforting message:

mysqldump: Got error: 1044:
Access denied for user 'root'@'localhost' to database 'information_schema'
when using LOCK TABLES

Of course, the error is doubly strange.

• First, because mysqldump is not supposed to attempt dumping the information schema. I know for sure, since I had to write a special script to dump the I_S when I really needed to export its data.
• Second, because there was actually a bug affecting this behavior, but it was fixed over one year ago.

Looking at the bug report, though, I saw that someone was complaining about this bug again recently.
I don't believe without action, so I rapidly installed a 5.0.45 and I tried to repeat the offending behavior. No luck. Everything worked as expected.
Then it dawned on me that perhaps he was not using the right mysqldump. I asked my friend to report the version of the dump, and indeed, it was a version that comes with MySQL 4.1.
Apparently, an old version of the client tools were still stuck in the OS execution path, before the directory where the new ones were installed.
A couple of minutes later, having cleaned the installation, all went well.
Lesson learned: before crying "bug", try "--version"!