The best description I can give of what's actually going on here is that {{formatnum:750000 [[Category:Articles with unsourced statements since January 2009]]}} formats both the number and the year in the category tag, resulting in "750,000 [[Category:Articles with unsourced statements since January 2,009]]". I've changed the bug summary accordingly, but I'm not entirely sure that this is a bug. Maybe it's reasonable to expect formatnum to leave things in link brackets alone.

robert.ribnitz wrote:

Yes, that is basically what is occurring. Making formatnum ignore things in link brackets (perhaps guided with an additional parameter) would probably be a suitable workaround.

I think the current behavior is acceptable given that only numbers are valid input for formatnum.

robert.ribnitz wrote:

While this is certainly true, it does not solve the problem. The problem is that the fact-template can also be added to "numbers", or "cells with formatted numbers" (in other templates). We therefore either need a replacement that can be used in such cases, or we need to fix the behaviour to be able to generate correct (ie. unformatted) references/categories.

It sounds like what you want to end up with is this:

{{formatnum:750000}} [[Category:Articles with unsourced statements since January 2009]]

instead of this:

{{formatnum:750000 [[Category:Articles with unsourced statements since January 2009]]}}

The use of {{formatnum}} where it is on those templates sounds like it's simply not leaving room for additional annotations -- the entire field is expected to be a number. This then fails to create the expected result when you pass in something other than a number -- such as a number and some other text which also includes numbers. It would be similarly broken if this wasn't a link and just said "{{formatnum:750000 as of year 2010 US Census}}".

Either the template should add a second parameter for annotations, or only the numbers should be given the number format wrapper, eg by applying it on the number in the template invocation.

See Bug 21054.

Related: T237467: Invariant failed: Bad UTF-8 (full string verification), which added a tracking category for this behavior.