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Wong notes that there is no forbidden knowledge in Kamar-Taj, only forbidden practices people are free to read the entire spell book as long as they don't perform any of the rituals. One thing I learned about cooking is, read the whole recipe before going into the kitchen.
The fact Kaecilius and Strange are irresponsible enough to even try shows their huge ignorance.
To be fair, no one should be trying to bend reality to their whims without reading the whole thing first. Think of it like those Communications tests in high school. It's possible these are translations/transcriptions of previous books that were too old to be handled, so the Ancient One/SS could have been testing/handling this stuff as they wrote/re-wrote and realized that if anybody OTHER than a Sorcerer Supreme tried this they'd get messed up.so the warnings came later to the effect "Well that was messed up, don't do this K?". Any sorcerer who's not such a Smug Snake would have read the entire book and know about the dangers. Strange and Kaecilius are just too arrogant to take the time to read the whole thing. Presumably, by the time a student (or even a Master!) is trusted enough to read those, they know where the warnings are. Both examples of this come out of the same book, which is one of the personal spell books of the effing Sorcerer Supreme.
note Oh, and I put in front of everything because that's where Strange suggests that's where warnings belong.
Though the curriculum at Kamar-Taj is implied to be about that lethal, given how the Ancient One strands Strange on Mount Everest because he can't quite grasp how to use his Sling Ring. Which of course means that someone just non-maliciously studying magic is likely to kill themselves if not make some big messes in the process of killing themselves before they figure that out. Strange nearly destroys time because he doesn't know that can happen! All I can imagine is that it's a security measure meant for the exact purpose it's put to in the film if someone's too eager for power to actually read the whole thing and just casts the spell, they get Hoist by Their Own Petard.
Why the bloody hell do the warnings come after the spells!? That's a sure-fire way to cause some serious accidents.