I can truthfully say every one of the following tips was learned by doing it wrong the first time.

 

1.  When setting wood posts, cut them to the correct height AFTER you have set and secured them in place...prevents the up/down affect of whatever is between the posts.

 

2.  When using an electric chain saw, always know where the electrical cord is...sparks and smoke happens quickly...

 

3.  Avoid the temptation of better wrench leverage by getting under a plugged sink when removing the waste pipe ...

 

4.  Never ever replace the oven element (even it looks like it just slides into a slot) without unplugging an electric stove.

 

5.  The flex input line in the cabinet under the sink blows first when the Water Heater thermostat goes out.  If you have pressurized cans there do not open the cabinet door, and you probably do not even want to be in that room, but open all home windows and get fans turned on quickly.