My bet is on a random bitflip more than an actual bug in the code, based on its localised nature and rarity.
Antialiased text always looks blurry to me after looking at pixel fonts all the time.
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Antialiased text always looks blurry to me after looking at pixel fonts all the time.
Not necessarily cosmic rays but things like marginal timing can cause errors like this, especially on GPU buses/VRAM that tend to have less protection.
GPGPU and now AI has made accuracy of results more important, but before that, GPUs were regularly ran at the limits and it was assumed that occasional barely-visible artifacts or otherwise computation errors whose results aren't noticeable were acceptable. (Imagine you're playing a 3D game and a few pixels in a frame occasionally have incorrect values, or some shapes are a pixel off --- unless the errors are massive, you're unlikely to notice.)