The goalposts are shrouded, not moving

Alice: Are there any examples of X? Bob: Here. Alice: Okay, but are there any examples of X'? Bob: Yes, here. Alice: That has feature Y, which- Bob: Dude, stop moving the goalposts. The thing is, I think in this situation, Alice often isn't moving the goalposts. Instead, she's looking for something, and not fully specifying it up-front. Maybe because she's speaking imprecisely. Maybe because she…