This isn't the first time a major bug has been introduced and found shortly after release. How do these major bugs get past testing? Does PHP have poor test coverage? This bug would have been found pretty quickly with basic unit testing.
Not identifying this bug was the result of an (in my opinion) egregious human error. PHP has decent test coverage, and this issue does cause the current tests to fail. I really don't know how this was missed.
If you read the bug report, looks like the tests were failing, yet nobody bothered to either test, or investigate the results...
Sounds like plain old incompetence. PHP is slowly becoming even more of a joke due to things like this.