Maybe it was always there and I didn't notice.
Are you looking at the matchups? After waiver wire processes if you opponent drops a starter for a waiver wire add that add goes automatically to his bench. For some reason Yahoo doesn't like seeing those matchup spots uneven so they add a empty bench spot to the opponent to even things out. You can't fill it though.
If you look at your opponents lineup I bet he has a starter missing. Count the total players and I bet they match up.