List of known bugs:

+ Large numbers of matches (e.g. %, g or v commands), with the
  ignorecase option set, triggers a memory corruption bug in the
  regex routines.

+ Autoindent doesn't work in the ex editor.

+ ^C isn't passed to the shell in the script windows as an interrupt
  character.

+ The command ":ab foo^J bar" prints a usage message -- non-word
  characters should be quoted in the underlying terminal engine
  so that the upper-level knows they're quoted and doesn't use them
  as delimiters.  (Note, this isn't historical practice, vi didn't
  permit escaping of ^J in this type of command.)

+ The options edcompatible, hardtabs*, lisp*, optimize*, redraw*,
  and slowopen* are recognized, but not implemented.  Options with
  an asterisk are unlikely to ever be implemented, so if you want
  them you might want to say something!  I will implement lisp if
  anyone ever documents how it really worked.

+ Screen repainting over slow lines, for some screen changes, is not
  as good as the historic vi's.

+ If an error results during input in ex, it is not displayed until
  after input mode is exited.

+ If the ex append command is used from vi, the input command buffer
  is overwritten by the ex_append function, causing random errors.

+ Colon commands longer than a single line cause the display to be
  incorrect.

+ When switching files in a small screen (O_WINDOW) with :e, the status
  message isn't displayed.

+ The usages of S_{REDRAW,REFORMAT,REFRESH,RENUMBER,RESIZE} are
  inconsistent, and should be reviewed.  In particular, S_REFRESH
  in any screen redraws all screens.

+ Historic vi permitted :g/xxx/vi, i.e. you could execute ex/vi as
  global commands.  Need to review all of the old commands to verify
  which ones could/could not be used as global commands.

+ If you run out of space in the recovery directory, the recovery
  file is left in place.