Situation You said an incorrect or too-general fore-blessing over food (even she'hakol--which does, b'di'avad, cover all foods, including bread, wine, the Five Special Fruits, and mezonot). Or you forgot to say the fore-blessing.
What To Do You must still say the more-specific, correct after-blessing, including birkat ha'mazon when appropriate.
Reason You must still say the correct after-blessing even though you said the incorrect fore-blessing.