I see a significant number of such errors. Over the past few months I’ve caught sentences in German where there were grammatical errors in the sentences themselves, like disagreements between person/number between verb ending and subject. I’ve found a few sentences in English that, if not grammatically incorrect, read awkwardly and seemed like something a native speaker would not say.
More commonly though I find translations that aren’t egregiously wrong but are misleading or somehow off. Like, maybe the meaning is translated correctly but the connotation is very different and not captured accurately. A common problem is differences in formality…more commonly, a highly formal sentence in one language will be translated into a highly casual one in English. Occasionally I’ve found English idioms used the wrong way, like I found one recently where someone used “laughed up her sleeve” (which means laughing secretly or behind someone’s back) to translate a sentence that meant “couldn’t stop laughing”.
The fact that I’m not really fluent in any language other than English makes me strongly suspect that there are far more of these errors than I am actually detecting. Yeah, if I pour over each sentence with a fine-tooth comb I could probably find more errors, but I’m mainly just reporting (using Clozemaster, and also commenting on Tatoeba) the sentences that I know well enough in the language in question to strongly suspect are wrong or at least less-than-ideal.
The thing that bothers me about Tatoeba though is not that there are errors, but rather, that they don’t seem to have a good quality-control mechanism. All I can do is comment on a sentence and hope that the owner fixes it. There is no way (to my knowledge) for me to flag a sentence as needing review, or unlink translations or flag a pairing of sentences as having a problem with it. And there is no way to see whether or not another user has flagged a sentence or pairing as problematic, all you can do is look to see if there is a comment on either sentence.