There are player's positions relative to a spawner in which either a visually active spawner never produces mobs or a visually inactive spawner does produce mobs.

What I expected to happen was...:
that the spawner generates mobs when visually showing activity (spinning mob and flames), and not spawning mobs when (visually) inactive.

What actually happened was...:
I found that there are are positions relative to the spawner in which the visual spawner activity and the actual spawning of mobs are inconsistent:

• sometimes the spawner shows no activity, but spawns mobs;
• sometimes the spawner visually appears active, but never spawns mobs.

This can be annoying when building traps etc., as the visual feedback of the spawner's activation is inaccurate.

Further investigation suggests that for the spawner to become visually active (spinning mob, flames), the eyes pos of the player has to be in range (from various sources and testing i assume that to be within an euclidian distance of 16 from the center of the spawner block). However, for the spawner to actually produce mobs, it seems to be necessary for the player's feet pos to be within this range.

(The center of the spawner block in the following examples has the coordinates x334.5, y76.5, z155.5.)

This is most obvious from directly above:
In picture #1, the feet at y=92.340 are just within range (76.5+16=92.5). The spawner is visually inactive, but you can see one of the blazes that just spawned from it. For the spawner to become visually active, I had to descend further until the eyes pos was within range (picture #2, y92.445).

It is less obvious but still plausible from besides the spawner:
In picture #3, my feet are the same height as the spawner's center and almost 16 blocks away along the z-axis. The spawner was visually inactive, but the image shows the spawn. Picture #4 has the exact same position horizontally, but is 1.5 blocks below. Now the eyes pos is in range, but the feet most probably are not any more (euclidian-wise). The spawner is visually active, but even after several minutes of waiting not a single blaze has spawned.

Finally, from below, which is why I originally stumbled upon this:
In picture #5, the spawner is visually active, but no blazes spawn. Note that the feet are out of range (y60.480; range would be 76.5-16=60.5). A bit above, with the feet at y60.519, blazes spawn (Picture #6).

Steps to Reproduce:
1. Go to a spawner, about 16 blocks directly above is most convenient; have a line of sight towards the spawner.
2. Descend until the spawner shows visual activity
3. Move upwards, one block at a time, until the visual activity ceases
4. Wait a bit: the spawner spawns mobs despite being visually inactive.

The screenshots were taken while using optifine, but I also checked vanilla: same thing. Also, it does not matter if the difficulty is set to easy, normal, or hard.

I am actually playing minecraft 1.4.7, survival single player. I apologize for mislabeling the version; wanted to put it on the forums but could not create an account. Actually, I don't know what's with 1.5.1, but I did not find anything related on the bug tracker or the snapshot release notes. Could someone please check with 1.5.1?