I've ran across this a few different times but finally remembered to note one of the tactics where it happens.
If there are two of the same piece that can move to the same square, Tactics Trainer on Android will allow you to move either piece and list the correct move. That will cause you to fail the tactic at a later move.
On the Android app, it will allow either Rfe1 or Rae1 but Rfe1 is the correct 1st move. The online TT only allows the correct move. When it allows Rfe1 it will then allow the rest of the moves but since there isn't a Rook on f1 anymore, you can't take the bishop on f4 so all choices on the last move are incorrect.
I've ran across this a few different times but finally remembered to note one of the tactics where it happens.
If there are two of the same piece that can move to the same square, Tactics Trainer on Android will allow you to move either piece and list the correct move. That will cause you to fail the tactic at a later move.
The example I came across today: http://www.chess.com/tactics/server.html?id=120210
On the Android app, it will allow either Rfe1 or Rae1 but Rfe1 is the correct 1st move. The online TT only allows the correct move. When it allows Rfe1 it will then allow the rest of the moves but since there isn't a Rook on f1 anymore, you can't take the bishop on f4 so all choices on the last move are incorrect.