One other thing: if you smoke, residue can also reduce the effectiveness of cooling systems and affect system performance.
EDIT: Not suggesting this was the OP's fault, just suggesting there are logical ways to reduce this risk.
No one has done a real root cause analysis and research shows humans are really bad at it when relying on intuition alone.
Cascading failure here is possible. I've never seen one actually light a battery on fire. The last encounter was a gaming level video card plowing a capacitor, starting a fire within the entire desktop.
From the article:
> If you have a MacBook be careful leaving it unattended on the bed, battery fires burn hot and fast with little time to react.
If this person was in a habit of leaving it in a poorly ventilated place, like their squishy bed, the electronics could have become compromised over time.
Also from the article:
> however with the newer unibody laptops there is no way you can see this swelling happening.
This is not true. Users have reported that prior to discovering battery swelling, their trackpad quit working and in some cases began separating from the top chassis.