Here is presented the tests results and the lessons learnt concerning an opto-mechanical device to scan the GREGOR telescope field of view. The scanning is done by means of a set of mirrors and a mechanism which allows to keep the optical path length constant, regardless the portion of the field being scanned. This system is intended to feed a static image slicer used for solar observations. The tight level of tolerances required makes its design and tests a real challenging activity which produces a lot of unexpected lessons to learn. The story after the issues detection, the consequent root cause analysis, the additional tests and tools developed to study the phenomena, and the construction of the solutions and issue mitigation mechanisms provides a good background to elaborate some recommendations for future developments.