All slate is not the same
There are a very large amount of different slates in the world and its a bad idea to just buy some without doing some research first. Slating a roof is an expensive process and you want to be sure to get your moneys worth.
A bad headlap can cause a lot of issues on a slate roof including leaking. Very often you see roofs with less than two inch headlap, no headlap at all or even sometimes negative headlap. If headlap is done wrong you may end up needing to re do the entire roof.
Bad Flashing Work
Flashing and slates are the two things that keep water from penetrating the roof. The flashing metal must be of adequate type and gauge but installing it should be done by someone with experience or training to ensure a good leak-proof roof.
Walking on the Slate
This is a very big issue that not many people realise or think about. Slate is not meant to be walked on. This will end up causing the slate to fall off so it should never be done except as a last resort or in unusual situations.
Lack of Information
If the builder has not done any research at all and just blindly rushes on with the job there are bound to be a multitude of issues. Slating a roof is not particularly complicated but it must be done properly there will be a number of issues further down the road.