Thermostat housing

You may have remnants of the old gasket or seal remaining stopping the housing from sitting flush, and make sure that the seal or gasket has been replaced. Just remove the housing and re check everything. Also, make sure that you did not crack the housing when installing and tightening. Pressurize the system and watch for a leak from the housing, you should see if it is from the base or the actual housing.

Piece of old gasket material that got overlooked. Old gasket sealant built up in the bottom of the bolt holes, preventing the bolt from being tightened to provide enough clamping force. Corrosion where the thermostat's outer edge sits, holding it up and preventing the housing from being clamped tight. Scratches or pits in one of the sealing surfaces.

A lot of mechanics use gasket sealer from a tube on both sides of the paper gasket. That can seal scratches and pits, and you can watch it squish out when the housing bolts are tightened. You only need a light film. That way it will only squirt out when the housing is tight. Too much sealant will still do the trick, but it is going to ooze out even if the housing is not tightened fully.

Unlike with valve covers and transmission pans, when sealant from a tube is used for a thermostat housing, it must be allowed to cure for a few hours before running the engine. The cooling system can be filled right away, but since pressure builds up in the cooling system, uncured sealant can be pushed out by that pressure, resulting in a leak.