Normally you'd post a FixMyPrint issue where the OP asks for a solution. Here instead I post an issue and the cause/solution immediately as well, hoping that it helps others.
This issue looks this way in a CoreXY printer, on a Cartesian printer you probably have only one axis looking weird.
The top part of the image has two printed objects in it, they're corner blocks for an enclosure I'm making. The right one has a bend in it. The toolhead would consistently move 1mm to the right or left at the exact same location. Normally a layer shift occurs due to warping or loose belts or hitting the infill somehow.
The left block is what it's supposed to look like.
Upon closer inspection of my belts it appears that one of them was stretched a small amount. These are cheap generic belts from Aliexpress that did last quite a while but were probably not the highest quality. At some point the filling material breaks and the belt stretches by a small amount. When that part of the belt goes over the stepper motor you get a layer shift.
Now I got Gates belts and hope that they last longer.
