This application note from SiTime gives practical guidelines for the effective probing of oscillator output, shows common mistakes and explains how to identify and avoid potential probing issues. Link here (PDF)
Electrical engineers often use oscilloscopes for purposes ranging from checking simple, lowspeed digital signals to accurate waveform and jitter measurements. They need to use probes to directly access arbitrary signals on a PCB. Probes can, however, put extra load on a signal or distort the waveform displayed on a scope. Therefore, probing should be done carefully.
from Dangerous Prototypes https://ift.tt/2WDCtt7