No joke. Here’s a snippet out of an E15k manual.

Internal SRAM protection inside the UltraSPARC III Cu CPU. With higherdensity CPUs and lower-core voltages, SRAM cells have become more vulnerable to bit flips from cosmic-ray disturbances. Single-bit errors for the majority of the internal SRAMs are detected and are recoverable.

I’m sure it happens. I just find it funny that Sun has to design its chips to withstand interstellar high energy particles (or what-not). I’m no physicist.