I hope his bugpost is not considered a suggestion, if so, I will of course accept that decision.

The bug:
Some block entities use the same GUI base texture.

While this is more efficient, as it saves some kilobytes, it affects those resource pack makers who want to retexture each GUI individually.

Block entities which share the same GUI base texture (potentially not complete yet, I'm still looking into it)

• Dispenser + Dropper
  
  
• Single Chest + Ender Chest + Barrel + Minecart with Chest + Double (Large) Chest
  
  
  
  
  

Side note: Relates to MC-116465, as all GUIs are not consistent towards each other, as for the used distances/sizes on all GUIs, and especially all block entities which use generic_54._png are offset, as the base GUI texture got apparently a wrong size.
That means in case Mojang would consider to split up GUIs, all current GUIs would have to be reworked before doing so; "perfect GUI" pack will be eventually provided, on MC-116465.