I turned the switch off and then proceded to change the light fixture and got knocked on my butt.----WHY?

The power wire may be originating in the fixture's box and then a pair of wires is run down to the switch so the hot wire can be shut off. That means there is always a hot wire in the box with the fixture that is feeding the fixture and maybe continuing on to other outlets. That wire is live and if you contact it that would be your problem. You should get one of those little buzzers that detect live wires. When you bring it near one it glows and buzzes. If it does not detect any live wires in there you would be OK. They are about $15 and could save your life.



If you are unsure which way the wiring is going, shutting off the breaker and then testing with the buzzer is the most prudent way to proceed.
I turned the switch off and then proceded to change the light fixture and got knocked on my butt.----WHY?
you have to turn off the main breaker to the house before you do any wiring. you can turn off the breaker for just that room, IF you are sure it is the right breaker. turning off the light switch on the wall breaks the connection between on and off but does NOT turn off electricity to fixture.
I turned the switch off and then proceded to change the light fixture and got knocked on my butt.----WHY?
Because the switch only kills one side of the circuit, When you are doing electrical work, you should ALWAYS remove the fuse of click off the circuit breaker. The power can KILL you! Be safe, if you do not know what you are doing call a professional.
Because you didn't shut off the power at the circuit breaker.
Because there was still current flowing to the device. Always uplug the fixture or if it is hardwired, like a ceiling fixture or a wall plug or switch, flip the breaker off. Even after I make sure there is no power to the fixture I check with a meter to be sure.
Most likely the hot wire was connected directly to the fixture and the neutral wire was used at the switch. So when you touched the hot wire and then grounded your self... well you basically got tazed for a second
With house wiring there are three wires. Hot, Neutral, and Ground. It sounds like the person that wired the switch wired the switch with the Neutral rather than the Hot.



If the light switch is wired useing the neutral then the hot is constant. You can produce a %26quot;shock%26quot; with the hot and either the neutral or the ground.



So if the neutral was switched and the hot was present at the light fixture all you had to do was become grounded for you to become shocked.



The proper way for the switch to be wired is for the switch to switch the %26quot;Hot%26quot;.
Usually light fixtures are wired so that power is only delivered to them when the switch is on. This is especially true in newer homes where it done that most of the time. However, in older homes it was very common to run hot wires to the fixtures and break the neutrals. There are certain applications where they have to be wired that way or the circuit won't operate as intended. Turning off circuit breakers is a good idea, but nothing beats a Multi-meter or some other type of tester to verify that there's no juice to the circuit you're working on. Even if your box is labeled correctly, it's easy to flip the wrong breaker. Guess there's a reason they license those rascally electricians.
Incoming power can be going through the fixture before it goes to a switch.
either this was on a three way switch, or your light isn't switched on the hot side and is switched on the neutral, or one last possibility is you got ahold of the wires in the junction box the switch leg was run off of. The best thing to do is to by a 15 buck voltage tick from Lowe's it will beep when ac voltage is present.
did you unplug the item? any fool knows not to work with an item that is plug into an outlet!