The Bug

Introduced in 16w40a (1.11 snapshot). Before this snapshot, if the shield was disabled, it would not play the Item breaks sound.

When blocking an attack of a mob/player that disables a shield (e.g. axe or warden attack), the Item breaks (item.shield.break) sound while the Shield blocks (item.shield.block) sound plays, even if the shield does not break. This is clearly seen when subtitles are enabled.

The Item breaks sound may be intentional to signify that the shield is disabled. In that case, changing the subtitles to describe that event would be better (e.g. Shield disables).

How to Reproduce

1. Enable subtitles by going to Options > Accessibility Settings > Show Subtitles: On.
2. Summon a vindicator and hold a shield in one of your hands.
3. Hold the shield up to block the vindicator's attack in Survival mode.
4. Note how when the shield blocks the attack, the Item breaks sound also plays with the Shield blocks sound. Trying this with a warden yields the same results.
5. If you try this with any mob which does not disable your shield, the Item breaks sound does not play when it attacks while blocking.

Additional Notes

Related to MC-98316 if the Item breaks sound is intentional.