I have definitely made mistakes in home repairs, but haven't tried house flipping and don't plan on it for quite a while.
Something easy that is required in most states while changing a water heater; be sure to buy your bottom drain pan before adding your new water heater.
Most homes will already have a small pan that fits at the bottom (under) the water heater...ours didn't

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Another small possible mistake you may incounter before the air conditioner is turned on for the summer; be sure the coil's 'moisture collecting pan' drain hole is not clogged before you turn the air conditioner on.
This is usually located near the top of your inside blower unit; a pan with a drain hose connected to it and the hose is going through the floor.
We managed to ignore the fact; you should clean this drain pan, and drain hole, before you turn the heater on for the cold months.
(We use a quarter cup of Oxi Clean mixed in a gallon of hot tap water; it will usually do the trick for cleaning the excess gunk and rust in the pan, and help clean out the drain hose too.)
Well, after a day of the air conditioner running on a hot day, we found a soaked carpet in our hallway.
This was caused by the drain hole being plugged and the water was not draining properly (basically slow dripping).
It took us a couple days to soak up the water and dry the carpet, plus the fact that our floor boards may now be destroyed (from who knows how much water that drained onto it).
I hope this helps someone keep from having a couple minor problems, turn into major ones in the future.