Some are obvious but cut me some slack, I typically need it.
Jegs and Summing brand SS Braided fitting parts can't be interchanged.
China SS hose clamps are garbage. (Barely had any torque on this one)
China Brake switches are junk. And difficult to get to after everything is assembled. Beating on it with a screwdriver is the only way it functions.
What did you LEARN?