"Pogo" is a dangerous behavior in launch vehicles caused when the vertical thrust couples with the inertia of the propellant. This sets up an instability where thrust pushes more propellant than requested into the combustion chamber, which surges, creating additional thrust as well as a backpressure that momentarily opposes additional propellant entering the chamber. The instability can get worse and worse over time, leading to vertical vibration in the rocket itself. Hence, "pogo," accelerating as though on a giant pogo stick.
This is extremely hazardous for several reasons. First, rocket engines are designed to work in only a narrow range of mixture ratios and flow rates. The pogo effect moves you away from the ideal mixture in an uncontrollable fashion. This leads to less efficient combustion, which can quickly lead to heating or erosion of the rocket components themselves, in turn leading to a large, smoky fireball.
Second, the fueling systems upstream of the rocket engine are extremely high performance and fragile. Ever have "water hammer" in your house? Imagine it in plumbing that carries 100,000 gallons a minute, fed by boost pump turbines running at 200,000 RPM. Oh, and the fluid is at cryogenic temperatures. And unlike water, it readily explodes.
Third, the vibration can cause secondary damage in the rocket, and structural vibration coupling can exacerbate the pogo effect even more. Some failure mechanisms are "hard," like rivets coming loose or tanks buckling, but there are also "soft" failures like navigation sensors becoming less accurate and thrust vector control being less precise, which can in turn increase aerodynamic loads and heating on the vehicle.
The Saturn rocket programme implemented many fixes to its rockets to fight the "pogo" effect, most having to do with careful balancing of pipes and inclusion of complicated pressure reservoirs and self-regulating systems. This is part of the cost of doing business in space -- there are still a lot of things we don't know about building good rockets, and quite a few of those things can be deadly. The most famous example was on Apollo 13, where pogo grew so severe that the center engine of the second stage automatically shut down during the burn. Shutting down a rocket engine is a perilous thing... but we got away with it.
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After reading this thread, I see no evidence that the poor nut who initiated it has any ability to comprehend what the rest of you are saying. It's rather sad.
