Bus contention: it's not a bug, it's atmosphere
On the ZX Spectrum 48K, screen RAM lived at $4000–$7FFF — memory the ULA video chip needed to read to paint the picture. If your Z80 code also ran from that "contended" region, the CPU and the ULA fought over the bus. The ULA won by injecting whatever byte was floating around, splattering garbage pixels across the display: the legendary snow effect. Coders learned to keep hot loops above $8000. You, however, are here to cause maximum chaos. Uncle Clive would be so proud.