The Bavli, Gitin 2 amud 2, says that one person is believed when he testifies about isurin (prohibitions), and it immediately limits that to the case that the person is not testifying against an existing status quo (chazaka).
Rashi's commentary indicates how we know that one person is believed:
for the Torah believed everyone about t'ruma-tithing [i.e. that he had tithed], slaughtering [i.e. that he had slaughtered properly], and removal of [forbidden parts of the meat, i.e. that he had indeed removed them].
Tosafos jump on this:
He should not have mentioned t'ruma-tithing and slaughtering, because someone is believed about those even though there is an existing status quo of forbiddenness, because he can [or could] fix them [and get rid of the status quo].
What about removing the forbidden parts of the meat, thereby fixing the overall meat they're in? He could do that, too. Why don't Tosafos object to Rashi's example?