Implementing a way to get some useful debug info about why a form was bl...

...ackholed
Using the log does not mean "write a file" :).

The proposed solution simply won't work with 2+ users using the same app as each user will write to the same location, which would lead to "Wat? Why is Cake comparing my login data to someone else's?! That must be the problem!".

In any event though, you're not using a file as a log mechanism, it's temporary data store - just use the session for that. _Log_ the result.