If you build (for example) a standard pipe, and change it to an insulated pipe without deconstructing the old pipe, the glass will keep warning about the risk of melting for the standard pipe that is no more in place.
This issue is solved by reloading the game.
Steps to Reproduce
Build a glass forge with an ouput pipe that will melt with molten glass. Then replace the pipe by an insulated pipe (igneous rock or ceramic) without deconstructing the old pipe.
This issue has not been confirmed by a developer yet.