What are some sources of error in this physics lab?

[Sheena]

The lab called on us to use a vacuum tube to determine an electron's charge to mass ratio. Here is a link to the lab: http://www.clemson.edu/ces/phoenix/labs/cupol/eoverm/index.html. What are some sources of error in this lab?
I already know that human error and error in measurement can skew results, but I can't think of any other sources of error. I have to write a full error analysis and I don't have many ideas. Any explanations of errors would be appreciated. Thank you!

 

[phinds]

What measurements are you making? What physical elements are involved in the measurement? Surely you can think of some related errors.