Insane Hydraulics
Bold and audacious blog about fluid power
The mistake lies in incorrect positioning of the chamfer of a rotary part that works in a blind machined cavity, and although it can happen to many components, I would name the Rexroth A4VG charge pump as the most common example, at least for a typical pump overhauling shop.
When a blind cavity is machined, the rotary tool that cuts it will always leave a corner radius, which is why the rotary part that works in this cavity must have a small chamfer on the corresponding edge.
General piece of advice here - when you come across a rotary part that works in a blind cavity and can be mounted both ways - make sure to inspect it for a chamfer. If the part does have it - it should be directed towards the "blind" corner.
Read this article if you wonder what could happen if you mounted such a part the other way around.