I just sent out the previous letter before it was finished, so here comes the rest of it (sorry about that):
Steps to repeat:
Evaluate the following:
(dotimes (x 10) (insert "abc\n")))
C-g (Just to know where you are in C-h l)
Now press TAB and down arrow over and over again, as fast as you can, like you're reindenting code.
After a few lines, Emacs stops to indent. Looking at the end of C-h l, you will see something like:
C-g <tab> <down> <down>
<tab> <down> <tab> <down> <tab> <down> <tab> C-h l
Here, a <tab> is missing, which explains the indentation problems.