I copy-pasted a random LaTeX doc from many years ago (I made sure I wrote it before 2020 so it did not use any new features not in TeX Live 2020), and upon compilation I got the error
Font OT1/cmr/m/n/10.95=cmr10 at 10.95pt not loadable: Bad metric (TFM) file.
on line 13017. The document had only 200 lines so this must be from a package. It was really not clear where the error was coming from. I guessed that it was coming from \usepackage{microtype}, so I commented it out, but the same error persisted.
I decided to hard refresh the tab and this time I got a different error:
Can't tell much from those two lines, but if you open an issue with the document, or just provide the full compile log, I could dig into it.
The TFM error usually means a font file wasn't available. However, the service worker validates the downloads before caching, so any error pages or empty bodies get rejected (which makes posoned cache unlikely), but a stale state may still be possible.
Hard refresh bypasses the service worker, I patched around that last month but apparently not well enough.
Also, better error logs and reporting is already on my ToDo-list, sorry for that.
I decided to hard refresh the tab and this time I got a different error:
on line 39. Again it was unclear line 39 of which package.