Motspur 

(n.) The fourth wheel of a supermarket trolley which looks identical to the other three but renders the trolley completely uncontrollable.

I was caught out by the motspur on my trolley and knocked over a baked beans pyramid at the shops yesterday.

Motspur by MrSplinch December 16, 2016

motspur 

The wheel on a shopping cart that looks totally normal.....but when the trolley is loaded makes the whole thing uncontrollable.

Look that cart is motspurred

motspur by Paul December 25, 2003

Motspur 

The pesky fourth wheel on a shopping cart that refuses to cooperate with the other three.

The motspur wobbled erratically as she pushed the cart down the aisle.

Motspur by McGluey September 25, 2008