Some people - including me to a small degree - have had issues with the X axis runner not being level. This appears to be chance on particular machines and is probably a QC problem. It can cause leaning (tall) prints, or in an extreme case, make it hard to level.
I put some thoughts down on this some time ago (they probably also apply to other models, but I only have an SV01) - http://www.nightshade-arts.co.uk/techdocs/_datasheets/SV01%20X%20axis%20leveling.pdf.