> Is there a common source of extra \r in malformed inputs, beyond those existing as part of \r\n?

Old Macs and some other systems use \r as their EOL, I still sometimes see that with string values in CSV files y code has to deal with (though I don't think I've seen it as an EOL marker in the format itself for a _long_ time).

Sometimes incorrect cleaning steps can leave them behind, such as replacing \r\n with \n but that replacement not being global: it tests fine on strings with zero or one \r\n but subsequent ones will retain their \r. Also code splitting on \n assuming it will always see just that as EOLs will leave trailing \r characters in place. Also, code cleaning EOLs from strings that are supposed to be one-line-only may replace \n (or \n or \r\n, ignoring the possibility of just \r) with a space or a comma and a space, that could be where the \r characters in certain string values I see in files from clients are coming from.

I suspect that off-by-one errors caused by character counting bugs in UTF8/UTF16 handling may cause splitting on EOLs to be a bit off in some cases, though here you will probably be seeing other data corruption at the same time and an errant \r is one of your smaller problems.