In Covenants it is mentioned that casting tablets are dangerous because the caster don't understand the spell's workings, whose parameters and details are defined by the tablet's creator, but that they let these who hadn't learnt "the spell intricacies through laboratory research" cast the spell anyway (if they don't fail the casting, of course).
With that in mind I'm thinking that given that Casting Tablets aren't particularly expensive they should probably be found around quite often. For example consider a group of just gauntled magi with no CrTe expert (neither time nor desire to get one soon) who wants to build a covenant, but don't want to sleep in huts until they get stonemasons and give them time (and stones, and money, and defense against the faeries, and so on and on and on): the problem would be solved spending a few build points in vis and a casting table for Conjuring the Mystic Tower, and towers could arise like mushrooms. Sure, the design would already be set by the author, but that doesn't have to be a bad thing (it is already hard enough to figure out a tower's inner design on your own, so probably casting tablets created by magi with good taste, sense of orientation and a knack for architecture and design would be quite popular. I can figure more than one or two towers created in the traditional way having chimneys going to nowhere, stairs too cramped to be used without bowing and so on).
So that makes me thing that if these casting tablets are popular, then it's quite probable that some magi had also been toying with the idea of making malicious casting tablets that take advantage of the fact that the caster of a casting tablet can't change anything of the original spell, and so with the bait of a nice effect can also deliver a malicious sting. How would those be?
My first thought, going on with the Mystic Tower, would be a tower hiding secret rooms, passages or entrances. If the casting tablet gets popular enough, when the malicious magus is visiting a young covenant and noticies a tower with his design (and sigil, next to the caster's), he would know of ways to sneak in and out of the tower, get secret spaces, and so on. But probably there are better things to hide within the tablet, not necessarily relying just on the spell casting details but on hidden effects or so. So can you think of any of these?