Recently I have convinced in it myself yet another time. The task looked as simple as generating load on a site accepting several files for parameters. As usual it was easy to record and parametrize scripts, debug and plan the first load.
The difficulties appear during the testing. Every time when I approach load testing the results kind of amaze me. They are not what I would expect. This time was no different. I was amazed and did at least 5 times more test execution than planned originally.
Thanks to my managerial experience I have lend myself some time for contingencies. So it worked well and I met the deadline :)
Every time you plan for load testing keep in mind that this is investigation rather than determined work in sense that we put in it when we approach planning.