The curse of tin whiskers rears its head again 

I've seen this issue arise with tactile switches a few times now. Unlike the normal failure mode of progressively failing open circuit due to oxide formation, this issue creates a gradually decreasing resistance across the contacts with molecular level tin whisker growth.
This issue seems to have originated from the time that lead was banned from solder, and the manufacturers ended up using pure tin to plate their contacts for solder compatibility of the terminal legs of the contacts.
It manifests as a very slow growth of micrometre diameter strands. With the super-low current of a tactile switch, it can progressively create a lowering resistance between the contacts that will start causing intermittent false operation of the switch that gradually gets worse over time.
The best fix is to replace the switch.
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