So after a super long hiatus I'm going to head back to my local "recently closed" quarry for some deep muddy fun.........that being said, I've only done a few sinking sessions there solo and haven't really experienced the full quarry experience if you get me ......getting to the point ..........any other experienced quarry sinkers have any tips of things to avoid ......I get heavy machinery etc ........but has anyone had any negative effects of getting totally covered in settling pond mud??? Skin irritation/ smell/ something they landed on while having muddy fun.... Kinda assume it depends on a case by case basis but would like to know before heading all out again .......thanks for all the tips and stay on the lookout for new free content from me very soon !!!

I would say 'watch out for the security cameras', and then, 'watch out for the security guards and maybe, even the local police'......

I would say watch out for any harms (->environmental/ personal/ social/ else) and don't panic if you are in there!
Also a good answer would be helpful, in the case you have to give them!

Watch for undetonated blasting caps and for holes drilled into cliff faces. Stay away from anything electrical. Don't jump into any place where you have not previously been to verify there are no hazards under the surface.

With security- be on the lookout for what looks like motion sensors, some of those are infrared 'lasers' and if anything breaks the beam it'll notify security to check the cameras.
(They are also very good at detecting deer- so every alert is presumed a false alarm until proven otherwise).

Stay away from fresh excavations or anything that may have been undercut- these are much more likely to shift or collapse.

Packing an anchor rope or and inflatable pool toy can make it easier to pull yourself out the mud.

How much cover do you have when you enter the quarry? Is the public footpath/access outside used a lot by dog walkers? (A phone-in from Joe-public is unlikely as you don't look like teenagers causing trouble).
If you can be easily seen walking about inside consider a high viz jacket and 'looking the part' (if something looks normal people don't notice).

Stagnant water (drainage ditches/ponds etc) may be hiding midges or other biting insets- nothing a dose of insect repellent can't solve as long as you're not rolling about in said water.

Plants such as Hemlock like wet and broken ground; not only a poison the sap can also cause severe (sometimes extreme!!) skin irritation. It looks similar to Cow Parsley, but it grows taller and has purple spots/blotches on the stem.
Same story for Giant Hogweed, Waterdrop Ragwort, and a few others.

Also beware of reed beds if the mud is being reclaimed or re-naturalised- some of them are remarkably sharp, I've a few faint scars on my legs from finding out the fun way.

I've had close calls and know now to never ever go on a weekday and if you find one of the large pumps running, then exact at any moment they could return to check it or check the fuel.