Open https://en.wikipedia.org/wiki/User:Neil_P._Quinn-WMF/T106905?veaction=edit, which contains the wikitext:
[[File:Green_Square.svg|right|thumb|An example image.]] A preformatted block.
Try to delete the preformatted block. If you select all the text inside of it and press delete, you can empty it out. However, if you then press delete to remove the block itself, it will select the image instead.
It's possible to work around this by pressing enter at the end of the <pre> block to create a new line below it, selecting the whole block from beginning of that following line, and pressing delete .