There are a lot of puzzles that consider alternative solutions as failures. Such as http://en.lichess.org/training/38554 (alternative is 1.Qf7+ and then Rd7 also mate in 4 as the solution) and many more.
In short there should be a button "report alternative solution" and special trained people with higher ratings (=moderators) could fix the puzzle or reject the alternative solution if it is wrong.
In short there should be a button "report alternative solution" and special trained people with higher ratings (=moderators) could fix the puzzle or reject the alternative solution if it is wrong.