HandlerSocket giving error 167 - what does it mean?

I have a PHP based multi-threaded application that I'm try to get working with the HandlerSocket plugin for MySql开发者_开发问答.

When running as a single process it works fine. When I start multi threading it, I get an error when inserting - it just says "167".

I have occasionally gotten "121" - which I've figured out means duplicate key error.

Any clues?

Thanks!

Apparently...

MySQL error code 167: Failed to set row auto increment value