If i were to measure the amount of more consumed over a given time at an outlet say where my hometheater system is plugged into. During that course of time I was constantly plugging and unplugging different devices into the outlet. Would plugging in different devices at different time cause errors in my measurement. Is this due to a changing power factor? Don't know much about power factor but have read a little on it. If it does induce error in my measurements what is the best way to combat it.

 

You haven't given us enough information to decide for sure, so the answer is "it's possible". If you're interested in measuring power and energy consumed, consider a Kill-a-Watt. They can be had for $20-$30 street price and do the job nicely, including taking the power factor into account.

You can make the same measurements with a scope, but the Kill-a-Watt is a heck of a lot easier.