It actually happens really easily.
Sets are built from dry materials. It doesn't take much to set a blaze. I worked on three films that suffered accidental fire damage. Fires are an obvious hazzard when dealing with fire elements on set, however, its usually other elements that cause the fires. The SPFX Technicians take extreme caution, they are trained professionals (most) are doing it safely.
It's the accidents that cause the fires. The set has an overhang that is too close to the fire. It gets hot while no one is noticing, until it is too late. The old Set Dec lamp, the wire is starting to overheat at the power outlet. It goes unnoticed until its too late. The painters painting the set in the rain. Desperately trying to get the paint to dry, set fire to the set by accident with the flame heat. The crew jacket that is too close to the propane heater...
As a Set Designer, keeping this in mind, adding notes of caution, pointing out the seemingly obvious is never too much. Even if it merely makes the crew aware of the potential hazzards, it is effective in fire prevention. A note above the header, and a quick illustration of the Set Dec Altar with flames shooting up, will be adequate to enlighten even a novice carpenter that they need to be concerned about potential fire. They would then address the issue with re design, or fire retardant, or both. Either way, without the communication, and pictures work best, the carpenters will have no idea there will be a fire burning 24" below the header. Not if its a Set Dec item. It could easily get overlooked. Until it's too late. And that's just an example. As a Set Designer you have a position where you can really make a difference in safety on set. Make notes. Point things out. Everything. I'm a big fan of illustrating drawings. In the real world a Draftsperson would be fired for it.
If you haven't watched the behind the scenes interviews on the second disk of Ridley Scott's Legend, you may want to. Its a really good behind the scenes story about how (in my opinion) one of the greatest indoor sets ever built tragically burned to the ground.