I've seen it plenty of times. Something triggers a JS execution bug that breaks the entire script. Suddenly, the site is dead. If you're lucky, it might start working after reload. If not, you'll have to wait until developers figure out something is wrong.

Causes vary. Sometimes it's a resource that fails to load. Sometimes it's a race condition. Sometimes, it's just dumb coding[0]. The more JS is put on site, and the more the site depends on it, the more likely it is to happen.

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[0] - Like one of the large food order sites in my country that won't let you submit a form if it contains a number in the comment box - the very comment box you're supposed to use to add details like floor number. The problem persisted for the last couple times I used them (in a space of several weeks), so I wonder if they even realized they have a problem and are losing business because people see the checkout form broken without explanation, and order elsewhere.

If you have a big website with lots and lots of different kind of visitors you either have:

- Visitors with a bandwidth so low it takes a long time for the JS to download completely (so making it work in the mean time helps a lot).

- Visitors with outdated browsers where your JS will stop on some unsupported feature you forgot to polyfill (I usually add a polyfill after when I see it but in the mean time it works), that's what I meant by "crashing", it never works 100% of the time in 100% of the cases, it depends on the browser as well.