DIAGNOSIS Improper loading or an unbalanced pitcher. The suction feet may also be dirty or worn.
FIX Place heavier, dense ingredients (like frozen fruit) at the bottom, closer to the blades. Add liquids on top. Do not exceed the max fill line. Ensure the pitcher is correctly locked onto the base. Wipe the bottom of the motor base and your countertop to ensure the rubber feet have grip.
IF THAT DIDN'T WORK Check that all four rubber suction feet on the motor base are intact and clean. If they are hardened or missing, the base will slip. Replacements can often be found.
HEADS UP Excessive vibration can cause the pitcher to unlock itself. Always keep a hand on the lid when starting a thick blend.