A linguist would argue that it's a mistake only if an adult speaker learned the language after the critical period and didn't manage to grasp all of the rules.
If a child (or someone who grew up in the language environment) made the "mistake" then it's an example of language change in action... unless the child is so young that it's simply a language learning artifact such as "I goed to the store today, daddy" (which, incidentally, is logically consistent with other past-tense forms yet ungrammatical b/c of social norms)...
If a child (or someone who grew up in the language environment) made the "mistake" then it's an example of language change in action... unless the child is so young that it's simply a language learning artifact such as "I goed to the store today, daddy" (which, incidentally, is logically consistent with other past-tense forms yet ungrammatical b/c of social norms)...