When devMode is activated, action result is modified to ERROR result if it is tried to set a non existent property via POST or GET parameter (or, in other words: there is no action/model property named like the given parameter).

While it is nice to be verbose to the developer on this issue, it must not interfere with page flow, such as having two different page flows in devMode and deployment mode for exactly the same inputs.

Instead, the possibly wrong parameter usage should be logged as a warning message, and maybe rendered as an action message (or ideally in a devMode specific debug box) if devMode is activated.