What is limp mode in PLL?
In this case a PLL in "Limp Mode" would suggest it's still outputting a signal as best it can, however it won't be phase locked like it should

Limp Mode or Limp Along Mode is often a feature used to make sure something will keep running even in the event of certain failures... often seen in cars and planes..

See http://en.wikipedia.org/wiki/Built-in_self-test

Quoting from the wiki

Automotive


  Automotive tests itself to enhance safety and reliability. For
  example, most vehicles with antilock brakes test them once per safety
  interval. If the antilock brake system has a broken wire or other
  fault, the brake system reverts to operating as a normal brake system.
  Most automotive engine controllers incorporate a "limp mode" for each
  sensor, so that the engine will continue to operate if the sensor or
  its wiring fails


Aviation


  Critical flight equipment is normally duplicated, or redundant. Less
  critical flight equipment, such as entertainment systems, might have a
  "limp mode" that provides some functions.