Session crossing error in portlet

Hi,

We have been using Pluto for integrating JSR168 portlet into
our protal product for years, recently our client reported some
strange session-crossing error: when several users accessed
one portlet at the same time, one user saw another user's session!

This happens rarely but does happen, currently we are not sure
the source of the bug, are you aware of any known issues in
Pluto similiar to this error ever?

Any info or hints will be greatly appreciated!

Lisheng