It took me a moment to find it, but an example of a car in which this happened can be seen here. If the fuel in the canister becomes sufficiently warm the pressure can rupture the canister. Usually the bottom everts first, popping outwards, although I have heard of instances where this was immediately followed by it coming apart, so this should not be considered any form of early warning measure. At this point you have a few ounces of liquid butane that's rather far above its boiling point and suddenly under very little pressure, so it almost instantaneously transitions to gas and vapor.
Such an event is called a BLEVE (boiling liquid expanding vapor explosion). Fire does not need be involved; in the referenced image above there are no signs of fire damage. If you are particularly unlucky, friction or sparks can ignite the cloud of vapor, possibly resulting in a fuel-air explosion.
How to avoid it
Don't park your car in the sun, or don't keep fuel canisters in it. However, on some trips this might be unavoidable. I suggest considering the following:
- For vehicles with a separate trunk, store the canisters in the trunk and not in the passenger compartment, which will be heated more due to the greenhouse effect of the windows.
- Find a way to insulate the canisters, such as placing them in an ice chest or wrapping them inside a blanket or sleeping bag.
- Depending on your area, caching them outside the vehicle may be considered. Of course, that also makes them more likely to grow legs and walk away.
- Sunshades, car covers, and cracked open windows may also help.