Obviously I never do this...

Then after the 2nd, 3rd, 4th, etc person doesn't understand it you realise actually it's you that's the idiot.

I think it's an interesting debate on the pros/cons of getting people outside the project to test your designs. Can you objectively test your own designs? I think so, I even think you can be more nuanced in the testing.

There's also an argument to say that you can be more innovative in the design if you know that you can test it's performance yourself/internally, and react to observed changes. Experiment to see behaviour and evolve.
External companies are more likely to react very strongly to the elements you know are more experimental and uncertain.

I've had some bad experiences with external testing companies and I continue to be dubious about the weight that is given to them by clients. I would rather they were labelled something like "another pair of eyes" or "opinion of a peer" instead of "1 day infallible expert usability review and final say"

(p.s. Leisa, I have to say, I've never had a problem with Flow.)