It doesn't have to be tons of open tabs; it can just as well be one tab that makes tons of accel draw contexts all on its own — maybe temporary hidden <canvas> draw contexts that it never cleans up. But a dev would usually end up hitting that kind of problem during development; whereas, if the problem never arises through "normal use" but instead requires having 100+ open tabs on the site to trigger, that bug might go unnoticed all throughout the QA process.

I can also extend my speculation about all the times I've personally seen "Error 5", although this is probably dipping into superstition territory: I think the per-toplevel-origin draw-context limit isn't static, but is influenced by the amount of VRAM available on the computer as a whole.

So on an extremely VRAM-constrained system like a Raspberry Pi (or on a system that's running Cyberpunk 2077 on one display and Chrome on another, sharing a GPU) the per-toplevel-origin limit on accel draw contexts might be dropped as low as, say, 5: an amount low enough that just having one extra .gif on the single tab of the site you have open might be enough to make the tab fall over.