To my understanding, in order to go to Limbo one basically has to die within I dream while being sedated such that one cannot wakeup, thus the mind has to remain somewhere else. But did the movie mention anywhere how and when Cobb and Mal ended up in Limbo? Weren't they simply dreaming around like everyone else? I can understand that an extraction gone wrong could have you killed in-dream while sedated, but it seems the two of them were dreaming together without any other (lethal) party involved...
Mal and Cobb appear to have been experimenting with the PASIV machine, trying to create dreams within dreams, presumably as part of their plans to go into business stealing corporate secrets.
What they evidently failed to realise was that it's possible to go so deep into the subconscious of a dreamer that it becomes almost impossible to get out:
COBB: We were on a job. Exploring dreams within dreams. But we didn’t understand how your mind can turn hours into years. How you can get trapped. Trapped so deep that when you wash up on the shore of your subconscious… - Inception: The Shooting Script
Ultimately Cobb was able (using inception) to convince Mal to escape at the same time as him by committing a simultaneous suicide on the railway. This allowed them to return to reality without either being left behind in Limbo.