The retina. It's like wet tissue paper. It cannot be sutured, but only manipulated with gas bubbles, silicone oil, or heavy liquids (perfluorocarbons).
Peeling diabetic tractional membranes off the retina, is like peeling an adhesive sticker off wet tissue paper - somewhat difficult.
Not only that, but the wet tissue paper bleeds.
You can stick it down with cryo or laser, but if the laser or cryo is too heavy, you get retinal necrosis & it re-detaches.
Many more intricasies...
Fascinating stuff though