Oh, indeed; I didn't notice the ability to open the inventory until you mentioned unlimited items. In that case,

Suggestion:
Display a hint stating "you can refill the items via the inventory" in case an item type (belt/underground) runs out.

Regarding the introduction-campaign:

Suggestion:
Have compilatron show an "Out of fuel"-warning on boilers similar to the "Out of fuel"-warning for burner miners and furnaces to notify new players to pay attention to this fact.

Regarding the introduction-campaign:

Suggestion:
Have compilatron show an "Out of ore"-warning on miners similar to the "Out of fuel"-warning for burner miners and furnaces to notify new players to pay attention to this fact.

Regarding the Campaign "Transport belt madness":
This bug applies to at least the first three levels of this campaign (probably all of them).

Current behavior:
When you hit the "Back to building"-button after hitting the "Start level" button, the amount of available items will be reset to the ...