Pogo oscillation is a self-excited vibration in liquid-propellant rocket engines caused by combustion instability.
If the pulse cycle happens to match a resonance frequency of the rocket then dangerous oscillations can occur through positive feedback, which can, in extreme cases, tear the vehicle apart.
[3][4] Pogo oscillation plagued the Titan II first stage during its development, which delayed man-rating the rocket for the Gemini program.
The oscillations during Apollo 13's ascent caused the center engine to shut down about two minutes earlier than planned.
The launch vehicle reached initial engine cutoff, but exploded 107 seconds after liftoff and disintegrated.
[7]: 440–446 Modern vibration analysis methods can account for the pogo oscillation to ensure that it is far away from the vehicle's resonant frequencies.