@HoukaiAmplifier99 I might have made this up, but I think I recall reading that Valve routinely licences old and weird codecs so that they can build support in Proton for some of these fringe cases.
The only time I can remember seeing it recently was in an old game off GOG called Conquest: Frontier Wars. Like others, it just showed a coloured pattern, but with that game it couldn't recover from not being able to play and would crash after.
@HoukaiAmplifier99 I don't remember my source, and I can't find anything searching around. I either made it up or it was an unsubstantiated reddit comment that stuck in my brain :)
For real instances of this problem though, look at Glorious Eggroll if you haven't already. Contains a number of additional video codecs Valve can't yet support directly.