Load testing your work

[Short.Circuit]

I got bit in the *ss, by making an assumption. Customer wanted
a couple outlets in the basement of old house. Found a j-box in the
ceiling nearby, meter shows 120V, sweet. Jumper down to 2 plugs.
Plug tester shows everything is cool. Get callback, power tools run
1/2 speed. Meter shows 70V when drill is turned on.
Loose connection somewhere in the circuit I tied into,
god knows where. I'll be more careful in future. rats

[brantmacga]

its generally a good idea to find out what is on a circuit before tapping it, and knowing what that circuit may be loaded to at any given time.

[LarryFine]

And to make sure you haven't grabbed a switch loop, placing your new load in series with an existing one.

[sawdust454]

http://www.tasco-usa.com/inspectorIII.htm
Use this tester and that will never happen again;)