Special:Block has a "confirm block" checkbox in some circumstances (at least when blocking yourself). This is small and non-obvious at the bottom of a pile of other checkboxes that you usually don't have to push, and is very far away from the big red warning that "You are about to block yourself! Are you sure you want to do that!"
Initially I couldn't tell that I was supposed to check a box and thought I just had to click "Block this user" a second time; after a few times through that I found the checkbox.
- replace the checkbox with a button
- put the checkbox next to the warning