This is standard description of a UAF (use-after-free) flaw. Reliability can vary from "30% of the time, it works every time" to perfectly reliable. It just depends on the instance of the bug. It is one of the most common exploitable bug classes found via fuzzing.

Because so many browser fuzzing crashes are UAFs, people have put a lot of effort into developing reliable techniques for exploiting them.

See e.g: http://www.rapid7.com/db/modules/exploit/windows/browser/ms1... for a reasonably reliable example.