Really hard to explain... but I was hoping someone might be able to help.
I have a mini i3 clone I built, and I've been able to get print quality quite good, however I noticed that along the X axis, any walls closest to the right side of the printer (where the belt tensioner is) turns out looking lumpy... Over extrusion maybe? You would normally assume that belt tension is loose, but I don't believe it is... and on the left side of the print, it's quite smooth! Not perfect or as smooth as the Y face, but significantly better than the right side of the print.
Some of you may be saying, "oh that's Z wobble!" but it is not. However I do have very slight Z wobble that you can see as a regular repeating pattern on the back side of my Y axis face, but as you can see, the irregular lumps on the X axis right side do not line up with the "threads" of my Z axis.