Mode of failure during dislodgement of prosthesis cemented with zinc polycarboxylate cement is usually seen at the:
LUTING INTERFACE

Luting cements are designed to ﬁll the microscopic gaps between a prosthesis and a prepared tooth. 
Each surface is microscopically rough, having peaks and valleys, and only point contacts are made by the peaks. 
The areas that are not in contact must be ﬁlled by the cement to prevent oral ﬂuid ﬂow and bacterial invasion. 
One can seal the space by placing a soft adhesive between the two surfaces, which can conform under pressure to the roughened surface features.

Ref: Phillips Ed 12th P: 311
Cement tooth interface
Cement prosthesis interface
Cleavage through the cement layer
Fracture of tooth or prosthesis