It's not great UI and it's made worse if, like me, you mostly read HN on a tablet so hitting the wrong link is easy to do. A better speed bump would be to have

It's not great UI and it's made worse if, like me, you mostly read HN on a tablet so hitting the wrong link is easy to do. A better speed bump would be to have the flag link always available, but have it take you to a page that says "you are flagging the following item: <comment/link> by <author>. You should continue if <list of reasons> and not <list of bad reasons>. [OK, flag it!]" This (a) draws attention to the exact thing you are flagging (the individual comment/link, not the thread) and (b) puts the confirm link somewhere else on the screen so you can't double-tap the same hotspot by accident. The advisory text is optional, but as others have pointed out, it's not really clear where the line between flagging and downvoting should be drawn.