plist-get currently contains this code:

  FOR_EACH_TAIL_SAFE (tail)
    {
      <check for success>
      tail = XCDR (tail);
      if (EQ (tail, li.tortoise))
        break;
    }

I don't understand why the last two lines are there. They're
unnecessary for proper plists; for circular plists, they result in
unintuitive behavior; and they depend on details of the
FOR_EACH_TAIL_SAFE implementation.

Can someone enlighten me?

As a tangential issue, shouldn't `equal' be symmetric?

(let* ((l1 '#1=(0 1 2 . #1#))
       (l2 '(0 1 2 0 1 2 . #1#)))
  (equal l2 l1) => t
  (equal l1 l2) => "List contains a loop" error.
  (plist-get l2 1) => 2
  (plist-get l1 1) => nil

Patches attached.