You know every so often you notice an actor, and then take a look at their profile on imdb, only to realise that the person you've been watching for xyz minutes/hrs/episodes was infact the same person who started in some cheesey role 20 years ago... you'd just never realised?
I'll give you an example,
Vaginia Haye:
http://www.imdb.com/name/nm0382162/
http://www.virginiahey.com/
Most commonly seen looking like:
Was actually that lass in Mad Max 2:
http://www.madmaxmovies.com/cast/MadMax2/VirginiaHey/
Keeping with the Mad Max motif, The Mouth of Sauron was also the helicopter pilot:
Feel free to add any more of your own, usually a b-movie secondary character turns up in a tv series or a supporting role in a movie decades later.