Hi!

Here's a recipe for the bug:
$ rm ~/.todo-do
$ emacs -Q
M-x partial-completion-mode
M-x todo-show
C-x C-b *scratch* RET
M-x todo-show
j Todo TAB TAB TAB

Error: PC-complete: Wrong type argument: sequencep, t

Okey, the problem is simple. `completing-read' is called with ("Todo"
"Todo") or the equivalent alist. PC-do-complete has a match with
"Todo", but it is not the only one, so he wants to find the common
prefix:
(setq prefix (try-completion (PC-chunk-after basestr skip)
                                                  poss)))
Sadly enough, try-completion returns `t'. Gosh.

I think that both files are to be fixed.  First, don't allow duplicates
in todo-mode: