I guess as a token answer I’d say anything dealing heavily with the death current. Some people will say there are demons, Goetic or otherwise, who will fuck you up badly if you don’t approach them the right way. I don’t really give much consideration to warnings like that.
In my case the most physically damaging was being kinda flippant/cavalier about my intent to overcome substance abuse. When I’ve done this in the past I’ve always very quickly manifested a path toward the outcome, but it’ll be something that is quite literally physically dangerous. Like I asked the djinn for help once with alcohol abuse and I ended up getting so drunk that I was hungover to the point of incapacity for the better part of 3 days and swore off alcohol (didn’t last and honestly it would probably result in death or hospitalisation if I went cold turkey).
Another time I used the Law to try and free myself from other substances, and what ended up happening (within about 10 days) was my supply was cut off via circumstance and I had to severely ration two (prescription) drugs down to like 1/5 the amount my body was used to, which, again, incapacitated me for about three weeks. Like I could barely walk, I was awake for 36 hours at a time, in constant discomfort. Luckily I had friends to support me with healing rituals, and it did result in me deciding to actually wean off everything, but in a controlled way and not one that could kill me.
The most mentally damaging was actually the best thing that ever happened to me on this path. I touched the divine and went through a dark night of the soul that involved a period of depression, self-imposed isolation, suicidal ideation, and feeling like I was completely unravelling mentally upon the realisation that reality isn’t real. But it hugely accelerated my understanding of the mechanics of magic and cut my manifestation time down to almost nothing. Like, same-day results in some cases.
I think safe work can be done in any system, but it relies on the thoughtfulness and intent of the operator. You can work “safely” even in “dangerous” currents if you put forethought into what you’re trying to achieve, and build towards outcomes.