Why are SAP error messages not displayed?

We have experienced a strange issue with GuiXT.
Some of the SAP messages appearing in the middle of the screen do not appear when GuiXT is switched on. The script for the logon page doesn't include any particular restrictions on messages.
We could see the message below only when GuiXT was switched off. (Embedded image moved to file: pic19951.pcx)
Have you experienced this sort of thing before?

The probable explanation is that you have a GuiXT script defined for this general screen, with an "Enter" command.
Sometime this happens when one writes a script for a different transaction, and adds a script for such a general screen, without using a

if Q[Transaction=...]

or a similar statement.