weird issue with pooled connections losing their user

[simon elliston ball]

I've got some strange behaviour with the new pooling. I'm using the keepalive tutorial scripts, running a large number of updates through a database. I then run some threads through a web server doing select queries as a different user. Almost as soon as the web server threads start doing their thing, the batch of updates get the error:
ERROR 1046 (3D000) at line 740680: No database selected
The server query log reports that access is denied to the database being updated for the user which was used for the selects. This is correct, the user doing the selects from the reading database should not be able to write to the writing database in this trial.

What puzzles me is how the writing connection (which is all over one mysql cli connection) manages to have its credentials taken away by the reading queries. Any pointers?