The first thing that needs to be addressed is: What is a bus? A bus is a communication unit that allows the CPU to interact with peripherals, there are different type of buses such as PCI, I2C, MDIO, Memory Buses, etc. Normally each bus would have its own protocol for transmitting data across devices, for example in the case of PCI we can have timeout errors or windows errors (data is directed to unknown addresses/devices). In memory, bus errors would refer to alignment but other errors could be attributed to physical HW problems such as faulty connections. Other type of bus errors could be single and multiple bit errors, this could be addressed by using ECC memory.