How do you use a timing diagram, and what's a real consequence of ignoring it?

A timing diagram in the datasheet specifies the temporal relationships a signal interface requires, setup time (how long data must be stable before the active clock edge), hold time (how long it must remain stable after), propagation/valid delays, minimum clock high and low widths, and the maximum clock frequency, with numeric min/max values, under stated conditions. You use it to verify your design honors those constraints: when bringing up a bus you read the required setup/hold and maximum clock from the diagram, then confirm with a scope or logic analyzer that your real signals actually meet them, for example checking that your SPI clock isn't faster than the peripheral's maximum and that data is stable around the sampling edge. Ignoring timing specs causes a particularly insidious class of bug: marginal, intermittent failures. If you clock a bus faster than the part's maximum, or your data doesn't meet setup/hold (because of slow edges, bus capacitance, or trace length), the receiver sometimes latches the wrong value, so the system works at room temperature or low speed and fails intermittently at temperature extremes, at higher speed, or on some units, which is maddening to debug because the logic looks correct and it "mostly works." This maps directly to the consistent-versus-intermittent debugging heuristic: timing-margin violations produce intermittent corruption tied to conditions. The fix is to slow the clock to within spec, fix signal integrity so edges meet setup/hold (shorter traces, stronger drive, proper termination), and re-verify on the scope against the datasheet numbers. So timing diagrams are the quantitative contract for an interface, read alongside the scope, and violating them is a leading cause of flaky, condition-dependent bus bugs.