They claim that they got both negative and positive results yesterday, which almost has to be user error or a software bug. Even if someone at the Serbian health agency had write access and were dedicated to helping Djokovic, why would they want to change the results a week after he landed in Australia? (A bad table join could easily cause this, although I'm not sure why that wouldn't have caused larger issues in Serbian test tracking.)

Even I made a mistake here in this thread and pasted Novak's url instead of mine. It's very easy to make user error when copying long urls with numbers in them. Especially when you want to check and recheck two QR codes.

I received seven digital covid-related certificates in two countries so far. 5/7 had blatant errors in them. Wrong vaccine, wrong place, wrong birth date, typos in name, completely empty, typos in the template. I was able to get them retroactively corrected where needed (3/7) and in two cases, the errors corrected themselves. Nobody ever looks at these things.