On page 79-81 it's not clear if the X-carriage should be attached to the rail before or after tightening the belts. If it's attached when pulling on the belts, it puts a fair amount of tension on the Z printing axis of the part, which can cause it to split.
fwiffo
commented
3 years ago 
On page 79-81 it's not clear if the X-carriage should be attached to the rail before or after tightening the belts. If it's attached when pulling on the belts, it puts a fair amount of tension on the Z printing axis of the part, which can cause it to split.
See this discord thread for more details.