I've never taken any computer based exam before so it'll be my first time taking it when I go take my GMAT. Just some questions for those familiar with the example:

1. Are MBA.com's GMAT practice tests and Manhattan GMAT practice tests of equal difficulty to the real one (including concepts tested)? I think I would freak out if there are suddenly much harder or material I've never seen before appearing on the real exam.
2. Did anyone here ever encounter sudden unexpected incidences during the exam? Computer malfunctioning etc... How do you guys deal with it?

Thanks.