Blenders should not leak your smoothie all over the counter. Luckily, there are only a few common issues that affect this issue with blenders.
Blade Assembly Failure
The bearing inside the assembly may fail from overuse and misuse — or bad design. The Ninja blender is known to form cracks radiating outwards from the blade bearing and will leak.
Some blenders, like the Vitamix, are designed for running at their max speed in order for the cooling fans to properly work. At lower speeds the bearing seal may melt completely or otherwise disintegrate. Water will then infiltrate the bearings during cleaning and wash away grease and cause rust to form. Do not let water sit in the blade assembly, as water will seep into the bearing. Wash and dry after using. Replace the bearings or entire blade assembly if the blade won't spin.
Cup Gasket Failure
A failing gasket can allow a leak to move past the lid and drip onto the motor housing.Inspect the gasket on the inside of the cup for wear. If the gasket is loose, cracked, or worn, replace the gasket.
Lid not Tightened
If the lid isn't tight enough, fluid will seep and leak out of the sides of the lid. Tighten the cup more before blending, and know there's help to open a stuck lid.
Plastic cracks as it sees impacts — like drops onto the floor — and as it ages. Using a dishwasher may accelerate the aging process, and warp the cup. Replace the cup.