MECLABS RAN a great web clinic last year on a collection of other threats to a test's VALIDITY beyond sample size. In it, Dr. Flint McGlaughlin ran through three testing errors and how to mitigate the risk of coming ACROSS them in your tests. I'd recommend reading the full transcript of the clinic, but here are a couple of errors from the list. 

History Effect: Something happens in the outside world to adversely bias your test results.

Instrumental Effect: An ERROR in your testing software undermines the testing results.

MECLABS ran a great web clinic last year on a collection of other threats to a test's validity beyond sample size. In it, Dr. Flint McGlaughlin ran through three testing errors and how to mitigate the risk of coming across them in your tests. I'd recommend reading the full transcript of the clinic, but here are a couple of errors from the list. 

History Effect: Something happens in the outside world to adversely bias your test results.

Instrumental Effect: An error in your testing software undermines the testing results.