Bad Ideas - Floors
Tinted varnish on wood. As it chips, light patches are seen on the darker floor, which looks particularly bad. Let wood mellow of its own accord, or stain it if a darker colour is wanted.
Vinyl on a damp concrete floor. Traps damp and grows mould.
Carpet in bathroom. Toilet aerosol is produced when flushing, and contaminates the carpet. Its not easy to clean out. Bathroom grade carpets are available, but avoiding carpet is preferred.
Patching damaged old floorboards with new boards. The appearance is very different, and it really looks bad. Better to use used boards.
Sanding boarded floors. It is not necessary for domestic floors, and sometimes exposes woodworm eaten cores, ruining the appearance of the floor. An hour of repeated washing is very effective, paint spots can often be scraped off, and spot sanding can be used to remove the infamous black stuff without removing any noticeable bulk.
Brick floors. Very difficult to keep clean.
No floor, that's probably a bad idea too.
Small or narrow pieces of chipboard in a chip floor. They just don't have the necessary strength.
1/4" plywood floor. It shouldn't need saying, but I've seen it done.
Replacing floors with concrete in timber frame buildings. It reduces the ability of the wood at floor level to evaporate damp away, and sometimes leads to rot. And rotten sole plates are not cheap to put right.