What is the main LIMIT on interleaving? When should you NOT interleave?

It helps for RELATED, confusable material — mixing totally unrelated subjects is just distracting context-switching. And build basic competence in a skill first, THEN interleave.

Interleaving works by training discrimination between things that could be mistaken for one another. If the items aren't confusable — say, French vocabulary mixed with calculus derivatives — there's no discrimination to train, and switching between them just fragments your attention. The benefit also assumes you can already execute each individual skill; if you can't do a single cone problem yet, shuffling cones into the mix only adds confusion. So the recipe is: reach basic competence on each type in a short block, then interleave to sharpen selection and cement retention.