Once in a blue while

A store owner was talking about his staff sometimes missing things and he said, “every once in a blue while they forget to do something.” This is a conflation of “(every) once in a while” (occasionally or sometimes) and “once in a blue moon” (rarely or infrequently). Maybe the speaker was thinking of a “blue whale” when he said “blue”. Who knows? A big thanks to Katie Minnock for hearing this one and sending it in!