What does Be careful what you wish for mean?

Answer 1

The rest of the phrase is understood--"Be careful what you wish for--you just might get it." What we think we want is not always what is best for us; sometimes we get our wish and discover the benefits don't outweigh the costs.

Example: Steve worships Mona from afar. He wishes with all his heart that she would notice him, fall for him, and be his forever.

Eventually, Steve's wish comes true. Steve finds that Mona is controlling, manipulative, jealous, and possessive. Oops.