I have had this occur occasionally with: E90, E91, E70 and F25. First time it happened it sort of freaked me out too. I had the hood open and was checking all the hoses for leaks or loose connections, etc.
It doesn't happen often, and every time it does I hate it... but it does not appear to be a problem.
What I don't like about it the most, is that it gets you used to seeing steam/vapor on occasion and if you did have a problem you might not check.
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I think it is bad luck to be superstitious
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