One of the parts of the bearing assemblies in the Y-Axis carriage supporting the heated bed on my RepRap Mendel broke after a few months of fatigue.
Blogged about it here: engineeringourfreedom.blogspot.com/2011/10/first-part-to-break-on-my-reprap-mendel.html

This thing reinforces all the parts with those little plastic square beams that are supposed to hold the bolts that support the printbed, by both adding a small triangle of support material to one side to act as a cantilever and remove some torque, while slightly filleting the edges underneath so that the stress is not so concentrated there.

Edited from Idaholion's solidworks models(http://reprap.org/wiki/Mendel/SolidWorks), which aren't correctly aligned to the print surface, so the individual STLs need to be rotated, otherwise take them from the blender file.