I have two concurrent users of my MacBook: one local (with local peripherals) and one remote (connecting via VNC to a user kept logged in with fast user switching).

As described here http://macosx.com/forums/howto-faqs/52547-howto-simultaneous-user-environments-via-vnc.html

That's working fine, except that when I hit modifier keys (e.g. shift, option, ...), I also affect the remote user.

For example, if I hold down shift, the remote user's key strokes are capitalised, and if I hold down option, they get strange glyphs instead of the normal letters.

Does anyone have any idea what could be causing this, or how to fix it?