Maybe all of those things. Given this issue isn't widespread AFAIS, it could have went through a bunch of tests and QA people, none of which caught the unique c

Maybe all of those things. Given this issue isn't widespread AFAIS, it could have went through a bunch of tests and QA people, none of which caught the unique combination of factors that might be rare but not rare enough to not cause problems on a lot of devices regardless. They should totally improve their testing methodology after such a bug, but I can see how a perfectly competent dev team could let such a bug slip. I haven't followed Signal's track record though, so I couldn't say if this is a one-off or a pattern.