The idiom for “trouble came from somewhere no one expected it to come from”

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Is there an English idiom/expression for "a problem came from a place no one expected it from"?

As in "we were not even paying attention to this part, because it was supposed to be simple & reliable, yet it generated the most problem!"

There's an expression in my mother tongue (Russian) that goes "oh comes the trouble from where least expected", is there an English version of this?

I would say "out of the blue", but this just means "unexpected" without stressing the "unexpected origin" of it.

UPDATE: I probably phrased my question poorly. I don't just mean "unexpected trouble". I mean "we were EXPECTING trouble, but we were LOOKING THE WRONG WAY"

Best Answer

The problem came from left field

come from left field.
To originate from a surprising or strange place. ("Left field" is the metaphorical origin of something uncommon, unpopular, or otherwise strange.).
That suggestion certainly came from left field! How did you come up with that one?

PS. I know this was originally a baseball metaphor, but we still use it in the UK where few of us understand or care about baseball!

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