A foreigner cannot make such an honest mistake. :) It is not something natural to say вървиме if you are not influenced by people who say аз вървим, ние вървиме. Of course, I am not saying that someone taught her that THIS is the correct form of all such verbs, I am just saying that apparently people don't always pay attention to teach Karolinka (or just talk to her in) proper Bulgarian.
When I teach foreigners some Bulgarian, I make sure that I do not teach them dialect.