My ex wife, many years ago was finishing taking a bath. She pulled to plug, and began to put her hand over the drain briefly, then remove it, creating an alternating gurgling sound.

"What are you doing?" I asked her.

"Making the water go down faster," she replied.

I asked her how she knew that would work. (She was a VERY bright person- 3.85 grade average, concert pianist, spoke several languages, edited manuscripts for scholars...etc.)

She said, "well, I have been doing it for years...it just seems to work!"

I asked if she had ever measured a precise amount of water and allowed it to drain with and without her hands assistance.

"Well, no..."
"Then how do you know it makes a difference?"

She thought about it a bit, and soon quit the practice, as SHE saw the point when I explained that the seeming increase of water flow from pressing down would be offset by the stoppage of flow when she covered the drain, and that I'd bet a measurement would show little difference.

We never actually did the experiment, but the point is, that even someone who is able to 'see' the flaw in the hypothesis(after it is pointed out) can be lured into a too-quick conclusion based on what they WISHED to see and using data extraneous to the test (sound etc,.)

It is very easy in experiments such as dowsing to miss some important factors when deciding what actually happened and when and why