When user interface elements unsolicitedly change their position, something unexpected might be clicked.

For example, the search engine list at the bottom of the Firefox suggestion box that appears when typing something into the URL bar ("This time, search with:"). When the number of suggestions changes, the height of the box changes, and since the search engine list is at the bottom, its position changes too. This could happen milliseconds before I wanted to click on one, which causes either searching for something I did not look for, or closing the URL bar.

It also causes an uncertainty whether the buttons will remain in place, causing delays until clicking.

The same happens on the image context menu for images on the Kiwi browser. It shows a preview thumbnail on top of the options in the context menu, but since the context menu opens before the thumbnail loads, the appearance of the thumbnail pushes the options down. If this happens few milliseconds before one is going to tap on something, it causes one to tap on the wrong place.