The bug

Using atlas tricks, the missing texture can be replaced with another image file in the resource pack.

According to the fix for MC-50073, replacing "missing" assets should not be possible.

How to reproduce

A resource pack is attached to this ticket that replaces the missing block/item texture with a white and blue checkerboard, and the missing GUI sprite texture with a red and white checkerboard.

1. Download and apply the attached resource pack

2. /give @s stone[minecraft:item_model="minecraft:nonexistent_model"]

3. /give @s stone[minecraft:tooltip_style="minecraft:nonexistent_texture"]

   , mouse over

Expected results

In both cases the standard black and magenta checkerboard should be used, since replacing that texture shouldn't be possible.

Actual results

The texture does in fact get replaced with the new checkerboard.

How to fix

Disallow "missingno" as a sprite name in atlas definitions.