Tolerances of components are things which need to be considered. A few months ago I was repairing another old 8-bit computer. After extensive testing and finding bits just falling off nowhere into the bus, I decided to check every TTL possible (around 70 sockets in). In external tester, everything was perfect.
I decided to replace all suspicious ICs with new ones - computer started properly and hanged a moment later. So, oscilloscope in hand and let's trace the failing bits. I found that one flip-flop developed a significant delay, and one passive circuit was not introducing needed delay to bus switch (8286). Maybe because of capacitance, flip-flop held its output state for a bit longer, what was totally needed to put the bit to devices in the switched bus.
Some circuits tend to drift in timings, but these timings usually are still in tolerances... Until two parts won't start to bend these tolerance regimes in opposite directions.