After calling IDirect3DDevice9::CreateQuery(D3DQUERYTYPE_EVENT, ...); I'm finding that alt-tab back to the applicaiton stops working - the window doesn't get restored correctly. This happens even if the query is never used.
Test under Vista 64, Catalyst 8.12 and 9.2 Radeon 4850, Core i7, 6GB RAM.
The only workaround I've found is to release it when losing focus, and recreate as needed, which shouldn't be required accoding to the D3D documentation.
Alas, many driver releases have their own quirks. The only way to workaround is to test of your application in as much software/hardware configurations as possible. Try to ask this question in the nearby topic ("GPU...").