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It just means it was flagged for some reason or another, or the user has below 0 rep and it will do it naturally.

In this case, I flagged him, but it is a legitimate user that was hacked but the link is stealing passwords.

Thank you, sir! Whew! I was worried.

yes, these are either flagged, or known fishing scams. Clicking on the "reveal comment" will just show the bad links they want you to go to.

Thank you!