Damaged push button

I've used a push button in one of my projects for a few weeks and while it still works, I noticed it was damaged pretty badly. I was just wondering if anyone had an idea what could have caused it:
(apologies for the horrific soldering, damage to the case happened during soldering)

General info:

The only damage I see is what you have inflicted while soldering. Did you consider using spade lugs instead of soldering?

That might have been a better idea yeah, never used those before though.
But the change in color above the solder made me quite worried, it doesn't seem normal at all...

You mean that left over solder flux?