The thing is - I have probably seen hundreds of projects that use tabs for indentation ... and I've never seen a single one without tab errors. And that ignoring e.g. the fact that tabs break diffs or who knows how many other things.

Using spaces doesn't automatically mean a lack of errors but it's clearly easy enough that it's commonly achieved. The most common argument against spaces seems to boil down to "my editor inserts hard tabs and I don't know how to configure it".