Is 1...d5 ever a bad move? I notice that as Black, I virtually always play d5 as my first move. It's usually in response to 1. e4 (Scandinavian Defense) or 1. d4 (Queen's Pawn Game).  As a hypothetical, if I premoved 1... d5, what is the worst that could happen? Are there any openings in which it is not a strong move?
Against 1.c4. This will allow white to exchange their c-pawn for black d-pawn and later gain a tempo. 2. cxd5 Qxd5 3.Nc3, or 2...Nf6 at some point will be followed with e4 with tempo. All in all, white can take control of the center.
According to lichess' database 1.c4 d5 is very rarely played.
However, the move is probably not losing; stockfish gives it: 0.6. The significance of this advantage varies a lot depending on the player's strength.