http://rr-project.org/ http://rr-project.org/ does it by recording the execution. If the bug occurs rarely, you can keep retrying your recordings until you get

http://rr-project.org/ http://rr-project.org/ does it by recording the execution. If the bug occurs rarely, you can keep retrying your recordings until you get one that exhibits the bug, and then you can step forwards and backwards through it at your leisure.