Thought you might be interested in some of the most common violations of our IEE Regs. in residential systems. Compliance with them isn't mandatory, but you'll see that most of these items are just plain bad practice. I don't claim this as a scientific study; just a few things off the top of my head that I see regularly.
1. Cables not securely clipped to ceiling joists, beams, etc. This has to be the most common violation I see. I've seen lots of houses where there doesn't seem to be a single cable clip in the whole place, cables in the attic being just loosely draped across the top of the joists.
2. Somewhat related to #1. Junction boxes not screwed to a solid surface, but just hanging on cables.
3. Outer cable sheath stripped back too far so that it doesn't enter the box.
4. Junction boxes with too many cables crammed in, usually where extra lights/outlets have been added.
5. Related to #4. Some wires pulled right back out of the sheath and spliced together OUTSIDE the junction box. Very common with earth/ground wires, which are sometimes just twisted together with no proper connector.
6. No rubber grommet used where cable enters a metal mounting box. (We don't have those neat clamps that you use for Romex - I wish we had something like that.)
7. Cables run too close to, touching, or even wrapped around hot water pipes.
8. Ring circuits which because of botched modifications are no longer rings, or which have spurs feeding several outlets.
9. Missing cross-bonding, usually after bathroom or kitchen renovations involving plumbing changes.
That should be enough to keep you thinking a while.