A simple job to test the lighting circuit as part of an EICR turned up this gem.

Gap filling taken to an extreme with the foam completely filling the light switch but nothing having changedexternally.

A short time after the EICR was finished the foam was chopped out circuit tested fully soon after.

Expanding foam found in light switch
Removal of the expanding foam in the light switch
Replacement of the light switch after the expanding foam has gone