Common Magic Arrows

What if you miss your shot?

  An enhanced arrow will lose it's enchantment and will become an ordinary arrow. It might still be salvageable as an ordinary arrow, but it cannot be re-enchanted or salvaged into usable reagents.
  An invocation is still going to explode. And it might explode on something or someone the archer didn't want to explode.
  The modern expression "Close only counts in horseshoes and hand grenades" might apply to fireball arrows, since it's a ten-foot radius, you might still wing your intended target on a failed roll.

Leave us Alone Mel Brooks!

  Invocation Arrows can certainly be used to catch buildings on fire and if you shoot enough of them at a wooden building, you will eventually set it on fire.   Invocation arrows like, invocation spells are really fash. The flash and bang is so fast the explosion that it often singes or flash burns wood without setting an ongoing fire.   It's up to the Game Master's discretion if a fire is started or not. In general any invocation can potentially start a fire but fireball arrows are generally the best at starting fire.   Dry hay, piles of parchment and the like catch fire pretty easily.   Generally if a invoker just wants to burn a wooden building down, using ordinary flaming arrows or torches will probably get the job done faster and certainly certainly cheaper than using invocation.   Hitting lamp oil or other accelerants virtually guarantees a fire. Some adventurers or high level magical saboteurs will sneak into an enemy camp a douse a target with oil or alchemist's fire and then hit with an invocation arrow at a set time later.