Setting a break on find-tail breaks everything at the repl:

  scheme@(guile-user)> ,break find-tail
  Trap 2: Breakpoint at #<procedure find-tail (_ _)>.
  scheme@(guile-user)> (+ 1 2 3)
  system/vm/traps.scm:127:31: system/vm/traps.scm:127:31: In procedure <: Wrong type: #f

  Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
  scheme@(guile-user) [1]> ,bt
  In system/repl/repl.scm:
     158:22  3 (_)
  In unknown file:
             2 (_)
  In system/vm/traps.scm:
     141:10  1 (apply-hook #<frame 7fff71aee2c0>)
     127:31  0 (_ _)
  scheme@(guile-user) [1]>

Not sure why, doesn't seem to happen with some other methods I've
tried... doesn't happen with map for instance.

I do have a patch, though I don't entirely understand what's going on.
Here's the relevant snippet of code:

  (define (frame-matcher proc)
    ;; [...]
    (let ((start (program-code proc))
          (end (program-last-ip proc)))
      (lambda (frame)
        (let ((ip (frame-instruction-pointer frame)))
          (and start end (<= start ip) (< ip end))))))

For some reason, `end' was #f in this scenario, which is how things
broke.  Well, patch attached!