When, say, an Iron Golem, attacks a Blaze, the Blaze will not fight back. So if it floats out of reach of the Golem, it will not shoot it, only float above it and then back down to be killed. This is quite useful for the player but it is not realistic; and can make the game quite boring. This does not work on players, which the Blazes shoot still as eagerly as usual!
Steps to recreate:
1) Spawn Blaze on survival.
2) Spawn Snow Golem.
3) Let the Snow Golem shoot the Blaze once or twice, but then block the Golem's line of fire with yourself.
4) The Blaze will rise above the Golem, but will not fire and soon will float down; and it will repeat this again and again, forever if nothing interrupts it.