Tests can't cover every scenario, no, but had they made a bit more of an effort to test realistically then it's absolutely possible that they could have covered

Tests can't cover every scenario, no, but had they made a bit more of an effort to test realistically then it's absolutely possible that they could have covered every scenario that mattered here. Over-reliance on unrealistic unit tests (which is likely what led to them not testing large packets) is a pattern I've seen cause issues like this many, many times before. I upgrade pretty regularly without reading release notes - relying on realistic tests to catch everything. What they do catch is usually not in the slightest bit obvious from release notes (often a regression in the dependency). Call it hubris if you like, but it works for me.