Measuring twice is still a thing.
Skipping the permits
Leaving wires exposed
Any electrical fix
Closing vents to focus heat on another part of the house
Squeezing space heaters into tiny spots
Putting new flooring on top of old material
Using the wrong replacement pipe
Using standard outlets near a sink or tub
Using all-purpose glues or tapes.
Fastening a deck to a home with nails
Painting over chipped paint or wallpaper
Replacing pipes in old homes without checking
When in doubt, call a pro.