Some textures, often thought to be unseen in game from the modeler, are easily seen and often ruin immersion on some things. A prime example is a Piglin Brute running, revealing the tops of their legs to be a dirty yellow and completely untextured. Some textures are more visible than others, but are still occasionally seen none the less.
List of affected textures
All piglin's top legs and bottom torsos (While moving)
Polar Bear's Top of legs (Unlikely to be seen)
Foxes inner tail and top of legs (Tail is seen easily in normal gameplay)
Insides of Sniffers inside their leaf fur (Completely red, seen if colliding into it)
Tops of Camel legs (While yeeting)
Inside of Frog's mouths (While eating, only 2 colors)
Top of Hoglin and Zoglin legs (While running)
Insides of beds (If broken and with any degree of lag)
Goats (While walking if one block lower)
Cats (Tops of legs, hard to notice on some variants so lesser priority)
Donkeys, Horses, Mules (Tails, seen on static model)
Evokers and Vindicators (Undershirt sides)
Pillager (Legs in general)
Ravagers (Numerous points, belly, back of head, likely more)
Blaze (Bottom of rods)
There are likely more, but the list would end up being far too long if I did.