IC housing and non-IC trim compatibility
Because IC housing are designed to be in direct contact with insulation they run hotter because the heat from the bulb cannot dissipate as quickly as a non-IC housing which will dissipate heat into the space above the fixture. Installing are vapor proof trim not approved for an IC housing, in an IC housing normally results in the plastic around the glass melting and dripping down.

With newer, cooler LED's and compact fluorescent bulbs this isn't a problem that much anymore but its still always better to match housing and trims per the manufacturer.