It seemed like such a good idea at the time: loads of grass trimmings from mowing, and trees to mulch. Grass clippings ought to make good mulch. The trouble is that they often harbor grass seed and create an encroaching wave of grass in the coming seasons. Not so good for new trees.
Or straw. Making a path behind the perennial bed, the straw seemed a perfectly good way to mulch and keep from packing down the dirt. Only if you want perennial grass weeds. The straw as a mulch brought in the most persistent and pernicious of weeds.
Shredded maple leaves. Leaf mold is the perfect soil conditioner. Shredded fall leaves seem the natural supply for a mulch that will enrich the soil while suppressing weeds. Only not if you are trying to grow anything that needs slightly acid pH conditions,like sweet gum trees. Maple leaves created a chlorosis situation. Now I simply compost them down and add them to cultivated soil.
Rocks to edge an herb bed, or surround trees, etc. ? Anytime you have to hand trim rocks is hard unnecessary work.