The chip designers are not that stupid, For instance the Philips design of the internal watchdog require 2 consequtive writes (01eh, 0E1h) to a SFR to refresh the puppy. Once it is enabled there is no way to disable it. It is much more likely that a toggle of an external bit (refresh external watchdog) will happen in a program gone astray than 2 consequtive writes of different data to the same SFR will occur.

Have fun,

Erik