Any language is a system of habits that tolerates variety (stylistic among others) while subscribing wholly to the rule of norms. We all speak our native language in such a way as to be understood by our fellows, and that means deviating from the norm sparely under rational circumstances.
This is by way of introducing the topic of what can only be called “willful mistakes.” For example, recently Y-H-B audited a series of lectures in typically fluent (but heavily accented) English by an eminent mathematician of South American provenience. For some reason, this person kept pronouncing the English word category incorrectly, with stress on the second rather than the initial syllable. Now, the same item in Spanish (categoría) has penultimate stress. Why, then, the constant mispronunciation of the English word? Even categorizing the mistake as “willful” (pardon the pun) only begs the question.
One attempt at an answer could lie in the level of awareness that we all exhibit to differing degrees of our actions, including speech. Speakers may be innocently unaware that they are exhibiting an idiosyncratic linguistic habit even when the specimen is unambiguously erroneous from the standpoint of the norm. This is true in spades when the language spoken is not native but foreign.
The question remains open.