Yup. Been there a few times.
The worst time was when I found out someone I cared deeply for had been actively lying to me [about beliefs] and... well, let's just say that THAT didn't last much longer.
I once discovered someone who'd been a family friend for many years had liked 'Britain First' on Facebook. I just couldn't be doing with that nonsense.
I know the feeling, it's not good, especially when they are people you've known since childhood. You feel your judgement of character is not so reliable as you thought it was. That Referendum stirred up lots of nasty stuff that was buried deep inside folks but had never seen light of day before.
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The worst time was when I found out someone I cared deeply for had been actively lying to me [about beliefs] and... well, let's just say that THAT didn't last much longer.