A common mistake is to reuse the fasteners that connect the motor to its load (e.g., tie two flanges together)
With a C-face motor, which mounts on the load, the step of connecting the motor to its load inherently takes place when you mount the motor. For most other motors, the output shaft must make a mechanical link to a gearbox or similar load. A common mistake at this point is to reuse the fasteners that connect the motor to its load (e.g., tie two flanges together).
When you torque a bolt to its specified torque (required for a secure connection), you physically deform the bolt. At this point, you create a tension that gives the connection its clamping power. The deformation, however, is permanent. A bolt typically loses 50% of its clamping power when you use it. So reusing it means you have a mechanical connection that's far weaker than intended, and it will probably not hold. You stretch the threads of nuts the same way. Locking washers are also single-use items. Note whether the fasteners you're using require a lubricant for the torque spec to be valid. Following the recommended torqueing pattern, connect the motor.
Now your motor is mechanically and electrically connected. Did the mechanics inspect the load and fix any problems contributing to the motor failure? If not, this one will fail also.