I get an error #610 - Maximum number of databases that may be accessed by a transaction is x.

A. There is also a very similar error 925.

Assuming the query doesn't really access more than <x> databases then this is a SQL bug caused by a complex query that has confused the SQL parser/optimiser.

It could also be that you're using more than 16 temporary tables - this message isn't very meaningful in this case. Do a showplan and see whether this might be the case.

Look for any known reasons for this bug at http://support.microsoft.com - just search on 610/925 and SQL and kbbug. If your problem does not match any of these and you have already applied the latest service pack then either post a reproduction script for an MVP to report for you or call the problem in to Microsoft PSS yourself.