Obviously the biggest concern of the GMAC for the online version is cheating. So anytime there's any sign of potential cheating risk -- they halt your test (with the clock still running) and try to address the issue.
Avoid the following:
1) Looking away from the screen or your whiteboard during the exam. If you have a tendency to stare elsewhere while thinking about a question -- that will trigger alarm bells.
2) Make sure no outside noises can be heard -- especially voices -- if they hear a voice they'll make you show them the whole room again.
3) Don't vocalize -- they will always assume you're talking to someone else
Do:
1) Pick a location which is completely private without interruptions or noise
2) Make sure the internet is rock steady
3) Tell everyone in your family / roommates etc to not disturb unless there's a fire (and only if its a big fire).
The easiest way to avoid trouble is to think like the person monitoring you -- what will trigger them to interrupt you.
I've had a number of clients who've been completely thrown off while they're on a role for something completely avoidable.
Just like everything on the GMAT, don't fight the reality of the online test -- adapt to it and stay within the lines.