Ah ha! This is just what happened to me today (and what I expected would happen to someone that makes every available mistake).
After compiling a new custom image and flashing the router with it, I found myself locked out of the router insofar as its OpenSSH server was concerned.
I promptly logged in through Dropbear and saw that the permissions for /root and /root/.ssh were wrong. When I used chmod to fix them to expected values, voila OpenSSH let me in!
Now so long as I have LuCI access and can re-flash with another (corrected) image, I am not truly locked out. But imagine how much more trouble to find out what needs correcting. Without Dropbear, every hypothesis would meant an image embodying it. (E.g. for every chmod, compile, flash, and oh let's see.)
What nightmare that would have been!