It could be practically anything; sometimes it's a little less pretty, sometimes it's a lot less pretty, sometimes a few little parts of the UI don't work as in

It could be practically anything; sometimes it's a little less pretty, sometimes it's a lot less pretty, sometimes a few little parts of the UI don't work as intended (maybe changing field X doesn't update field Y like it should, maybe button Z doesn't do what it should when you click it), and sometimes it just starts throwing exceptions like crazy and completely blows up. CSS is kind of sort of standardized, so you sort of have some hope there that all standards compliant browsers (not that any of them really are 100% compliant) will render the page about the same way, but once you get into Javascript land the myth of standardization quickly disappears, and multi-browser support becomes a never-ending nightmare. Unfortunately, you don't know which of those end results you're going to get until you actually try the thing out, and the answer could be wildly different on different pages of your app. It's still generally poor form to straight up disable a particular browser unless you know for a fact that it doesn't work properly (generally in the sense of you-can't-use-the-site, not things-look-kind-of-funny), but perhaps in this case they did in fact know that their app didn't work with Opera.