Linseed oil can spontaneously combust and start a fire even without an external heat source.
I just saw a news story about a guy who died when his house caught on fire as a result of a pile of rags that were soaked in linseed oil....he had used linseed oil when refinishing some wood in his house earlier that day and put the linseed oil soaked rags in his garage until trash day.
I had no idea that linseed oil could spontaneously combust. A quick Google search turned up quite a few news stories about this danger. ABC News did an experiment showing the spontaneous combustion in as little as 3 hours. The problem is that when the linseed oil dries, its temperature skyrockets.
Since almost all of us work with wood and are often looking to experiment and try new things, I thought that this is something that we should all be aware of. And apparently, there are some other products (ie. Teak Oil) that may contain linseed oil.....so READ the warnings listed on the containers.