Often these tests are automated since it's impractical to have a live person to check everything. But someone has to write the code the the regression tests (and test em too!), and that's another big, expensive task. It's pretty common for large projects to fall short of 100% coverage in regression testing. Since GW2 seems to get hit by these kind of 'unintended consequences' bugs a lot, my guess is that the problem is a lack of adequate regression testing.

This is perfectly reasonable, if you ask me. That is, both your analysis and the fact that the game lacks full testing.

I worked for nearly a decade as a software tester, and one of the projects I was on was for a jet engine controller. Given that this was a safety critical system (i.e., if the software fails people could die) we did a LOT of testing. We worked with the FAA to ensure that 100% of the code was tested, and any bugs were fixed or could be reasonably deferred until the next software iteration (and the FAA went through the deferrals with a fine tooth comb to verify that nothing was going to be a functional problem).

It took us almost three years to get one version of the software out for release. It took us another full year to get the next version out which was only a small iteration. And there was not a shortage of people working on this, it was a large scale project (I don't know exact numbers for either area, but I would not be at all surprised to know that the team working on the jet engine software was larger than all of ArenaNet).

Given the frequency of releases, and the size of the company, I think it's ludicrous to expect full testing coverage for GW2. Though I am still critical of things that are blatantly obvious (like "we removed this UI item" in the patch notes but the UI item is still perfectly visible).