It's a good idea but avoiding collisions with genuine passwords could be tricky particularly for sites with millions of accounts. If the honeywords are very long and random, then they probably won't collide but they will be obviously different from a typical users password. If you add a standard prefix or an extended character that is disallowed for real user passwords, that avoids collisions but also makes the honeywords easy to filter out. If you generate the honeywords by swapping around the characters in a real password then theres a danger that a user could set off a false alarm with a simple typo in their password.