As the AVR microcontroller family is running directly on the clock oscillator, the oscillator frequency for a specific throughput is relatively low compared to devices that divide the clock by 4, 8, or 12. This reduces the emitted noise from the oscillator, but the oscillator still will be among the noisiest parts of the chip.
High-frequency oscillators are quite delicate devices and are, therefore, sensitive to external noise.
In addition, the oscillator pins are generally more sensitive to ESD than other I/O pins.
Fortunately, it is easy to avoid these problems.
Keep the oscillator loop as tight as possible. Place the crystal/resonator as close to the pins as possible. Connect the decoupling capacitors (or the ground terminal of the resonator) directly to the ground plane. Even boards without ground plane should have a local plane under the oscillator. This plane must be connected directly to the ground pin of the microcontroller.
Care should also be taken when using an external clock to drive the AVR. If the clock source is far away from the AVR, the clock line will be a strong noise emitter and may also act as a receiving antenna for transients (and other types of noise) that may cause incorrect clocking of the AVR.
A buffer should therefore be placed on the clock line. A filter in front of the buffer will help remove incoming noise.