All actions in the game are made only after appropriate commands which are sent by the players. If you send troops to a target then they will surely arrive to it and not any other, as the game server performs the command which was sent from the game client. Your troops can reach another target only in case the server receives another command. Moreover, the commands from the game client technically cannot be changed during the process of its delivering to the server. The troops-tracking system is a highly-controlled element of the game-play. It is tested thoroughly at all stages by our QA team.
During a lot of successive attacks you can send your troops to a wrong target by mistake. So please make sure you have selected the correct target and troops before pressing the "Send" button.
Meanwhile, if your troops arrived to a different Prize, then do not be sorry. Anyway they will be counted into your future rewards on the Prizes.