This "bug" is triggered when the auto-reconnect feature reconnects to
the TS after a temporary network problem severs the connection.
Some suggestions:

1. Troubleshoot and fix your network issues if possible.  Under normal
circumstances, a network problem that breaks the connection to the
TS should be *very* rare.

2. Install the latest production TS client, version 5.2.3790.1830.  I was
not able to reproduce the unwanted popup behavior with this version,
after 7 attempts.  I was able to easily reproduce the unwanted behavior
with older versions of the client, for example, 5.1.2600.2180.

3. Disable the auto-reconnect feature.  This would give the user an
error instead of reconnecting to the TS.  You can set this in the .RDP
text file by changing the autoreconnection line to match below:

autoreconnection enabled:i:0

4. Use Vera's suggestion, as described in her TS FAQ:

http://ts.veranoest.net/ts_faq.htm#RDsession_popup

-TP