It takes time for a signal to travel through a gate. Gate Latency

It also takes time for signals to travel down wires. Propagation Delay

This can cause glitches.

Here is an example.

The XOR rule gives zero if the inputs are equal.

In this circuit, L and M effectively do nothing except introduce a tiny time delay.

Because the XOR gate inputs don't switch at the same time, short glitches can appear at Q.

Glitches can cause unwanted unexpected behaviour, especially in circuits that count pulses.