A dryer vent pancake isn't tasty at all! In short, it's what can happen when a company that cleans carpets and air ducts hooks up a truck mounted vacuum to a sealed dryer vent.
This particular customer had a vent that terminated on the roof of their home. When they moved in, they bought a $69 "Dryer Vent Cleaning" Groupon from an air duct cleaning company (that is no longer in business). This home was only 2 years old and had a dryer vent on the roof that had a damper that only opened when the dryer was running, which is exactly what they are supposed to have. When connecting an extremely powerful vacuum to a duct like this, it will suck the damper shut and create a seal that will ultimately collapse the ductwork itself in an attempt to draw out all the air in the system.
This ductwork was originally 4" round as it is supposed to be, when we arrived it looked as flat as a butter knife. Be careful who you trust to clean or repair your dryer vent!