Bukkit/CraftBukkit — comment on commit 5b36c2f96ca0a7a4d151ece412a9ff33cff6fd36
File: src/main/java/net/minecraft/server/Entity.java line 325
By EdGruberman:

Should this line not be:
EntityCombustByBlockEvent combustEvent = new EntityCombustByBlockEvent(damager, damagee, 15);

Otherwise it does not capture the block information in the event. I found I was unable to detect when a lava block combusted a player in a plugin by doing:
if (event instanceof EntityCombustByBlockEvent) {