The bug

Dispensers play the dispensing failed sound when they equip a mob head but it not affect wither skeleton head.

How to reproduce

•  Activate subtitles
•  Put a single wither skeleton head in a dispenser
•  Stand in front of the dispenser and activate it

-> It will play the sound when an item was successfully dispensed and show the subtitle "Dispensed item"

• Put only a single mob head except wither skeleton head in the dispenser
• Stand in front of the dispenser and activate it

-> It will play the sound when an item was not successfully dispensed and show the subtitle "Dispenser failed"

Code Analysis

Code analysis by SPGoding can be found in this comment.