Video: If you use a camera slider carriage for panning shots, the weight of some lenses can make the carriage tilt forward slightly and cause the carriage to bind up. You can fix that issue with a macro focusing rail. This video from youtuber thefrugalfilmmaker demonstrates a clever way to keep your heavy lenses from ruining perfectly smooth slider takes. Just add a macro focusing rail to your setup right on top of the slider carriage (he uses the DSLRKIT 2-Way). If the lens you're using is too heavy and causing your slider to bind, you can adjust your camera back on the focusing rail and re-balance your carriage slider so it glides smoothly from side to side.
Tip: Better Camera Slider Balance [YouTube]