When two (or more) different Juce-based plugins (I used the demo plugin and a blank plugin created with the Introjucer, both Release builds) are open (e.g. both GUIs are visible) only the last opened will receive correct mouse wheel messages. The others will always get a maximum increment (e.g. positive, no matter in what direction the wheel was moved).
This doesn’t happen in the 64-bit plugins, though I only tested 64-bit on our plugins.
I traced this back to juce_win32_Windowing.cpp:1956
return peer->peerWindowProc (h, message, wParam, lParam);
The wParam is already wrong, so it seems Windows is sending a wrong value.
OS is Windows7/64, VS2008EE, Juce tip.