When you are nowhere near lava/fire, even 5 blocks above it, it sets you on fire. This seems to hapen most often when jumping over it, however you can be nowhere near the ignite source and still be caught.

I encounter this often. You will be ages above lava or fire or not even directly above it, and you will catch fire. It will not act like you are IN the fire, but like you touched it for a split second. Tends to happen most when jumping across it or near it. Never had it happen when crouching over lava when it's right next to me. This is more annoying than anything but it did kill me once.