Lets assume a class A requires a class B which is defined in a different package than class A.
If you defined the wrong load order of packages (so you load the package with A before the package with B) you might easily hit the problem of "Undefined classes".
Pharo will now address this issue better by having first class entities representing missing (or yet undefined) classes. It is already planned for Pharo 7.
Here are the slides from ESUG 2017 on this topic to give you more informations.