This warning still appears as of 2022.2.15f1. I’ve been seeing this for what feels like at least a year, and I’m a little surprised that this might be the only place on the internet that mentions the bug. I went ahead and reported it to Unity as IN-39692.
Update 2023.5.9: Unity rejected my bug, claiming that they couldn’t repro it. That’s disappointing, given that the bug is so easy to reproduce for me. It’s possible I have some local setting that causes this, but I seriously doubt it. At any rate, I’m hoping other folks can help report the bug as well.
Here’s the content of my bug report:
What happened
If you select multiple particle systems in the scene hierarchy, and the particle systems have different sets of features enabled (e.g., one has “Noise” enabled and one doesn’t), then the following warning will be spammed to console:
Failed to store Builtin Skins/LightSkin/Images/ShurikenToggleNormalMixed.png > Builtin Skins/LightSkin/Images/ShurikenToggleNormalMixed.png
This seems to be an issue with the inspector attempting to display a mixed-state checkbox for any feature that is not consistently enabled/disabled across the selected particle systems. This seems to be confirmed by GUI glitches when you hover over the area where the checkbox is supposed to be.
How can we reproduce it using the example you attached
First, create three particle systems in a scene:
Particle system A: in the default state
Particle system B: duplicate of A
Particle system C: duplicate of A, but enable an additional feature, such as Noise
With the console in view, select each pair of particle systems. The following should occur: