If an external switch is connected to the RESET pin, it is important to add a series resistance. Whenever the switch is pressed it will short the capacitor, and the current (I) through the switch can have high peak values. This causes the switch to bounce and generate steep spikes in 2 ms - 10 ms (t) periods until the capacitor is discharged. The PCB tracks and the switch metal introduces a small inductance (L) and the high current through these tracks can generate high voltages up to VL = L * dI/dt.
This spike voltage, VL, is most likely outside the specification of the RESET pin. By adding a series resistor between the switch and the capacitor, the peak currents generated will be significantly low and it will not be large enough to generate high voltages at the RESET pin. An example connection is shown in the following diagram.
