The analogy is of a possum frozen in the headlights in the middle of the road. It should be noted that after much discussion and exploration of the concept, everyone came away with their own definitions, but the definition I came away with was: "Testing that is motivated by fear." An alternative definition was "Testing that you don't value" and yet another put a bet on both ways and used the definition "Testing you don't value, motivated on some level by fear." I'm sure that over time as we think and discuss it more, we will refine the definition.
(**Edit 30/06/2011 On reflection, I realise it's important to distinguish the "testing you don't value" aspect, so I see now that "Testing that you don't value, motivated by fear at some level" is the better definition.)
Possum testing manifests itself when a tester is undertaking any test activity, such as writing test scripts, creating test documents, and doing "bad" testing because of the fear of being wrong, fear of appearing foolish, fear of being exposed as ignorant, fear of being challenged, or even fear of losing your job. They are simply going through the motions because, like a possum in the headlights, they are frozen from fear to do anything else.
I can remember being a newbie tester, unsure of myself, and being told by people that wear ties that I need to script what I do so it can be repeatable. It seemed wrong to me, but I complied (for a day before I really decided that it was a complete waste of time) out of fear of being accused of not knowing what I was doing. Fortunately the road I crossed to become the tester I am today was mostly empty of speeding vehicles.
Our industry is full of possums, and it's no wonder. Testers are being confronted constantly by the blinding headlights of folklore and myth, attached to heavy process-vehicles driven by managers and even other testers who don't actually understand the complexities and subtleties of testing.
We need to take back control of our profession, and refuse to let the traffic freeze us. It can be scary. I recently was in a situation where I was asked to produce a document that I didn't value. Producing this document would have propagated deception in the name of process, but I feared the potential conflict that would arise if I refused. Realising that complying would be "possum testing" helped me to have the courage to take control of the situation, and unfreeze myself. I offered an alternative document that I did value, and I was able to step out of the headlights.
For those of us who have made it safely across the road, I think we should try to help our possum brothers and sisters who are still frozen in the middle of the road, and show them the true light.