At the time I wrote that I was referring to the general mental gear-crashing/freeze up when you've seen something the same way hundreds of times, the standard patterns are broken and your brain has to stop to figure out what you're looking at.

However with this now deployed on Meta, I can give you a better answer. I came across a page where this resulted in a particularly bad mess. I didn't foresee this particular result, and I'm betting you didn't foresee it either. In the middle of a several discussion sections there was an empty section. The result was two horizontal lines in a row, like this:

Comments
HeadingA

HeadingB
Comments

That is just plain bizarre, and I bet that it never occurred to you that that was going to happen. Two lines in a row is extremely unexpected and nonsensical. I expect many people seeing that will experience it a very distracting WTF mental trainwreck.

In that particular case the empty section was intended to invite discussion on topic HeadingA. However it could just as well have been a sample of article content posted on talk. Mangling article content posted on talk pages would be bad, therefore HeadingA is correct and the bizarre double-line is being caused by the inconsistent styling on HeadingB.