The biggest issue I've seen personally was when a vendor we were using would append query strings to filter data. On the second filter add, it would just break

The biggest issue I've seen personally was when a vendor we were using would append query strings to filter data. On the second filter add, it would just break in Internet Explorer, but Chrome and I think Firefox would work fine. Come to find out, their code didn't check the existing query string and was either adding extra # items or ? items, I can't remember which. Looking it up, though, the standard was that there is only one, so shockingly IE10 was the only one honoring it. The vendor, who works heavily in Chrome, was gobsmacked and their response was basically "well, yeah, but who uses Internet Explorer anymore?" to which we had to say that's fine, well, and good, but we must support IE10+/Edge.