yeah,
Reading the ability again, Lord Demon is right nothing special happens... you just shuffle the cards removed from game and place them back as your library. the effect of the original card happens.
You Play a 5 mana cascade card:
Cascade is on top of the stack.
Remove the top card from your library from the game (50cards left in Library) -> does it cost 4 mana or less "No"
Remove the top card from your library from the game (49cards left in Library) -> does it cost 4 mana or less "No"
...
Remove the top card from your library from the game (1 card left in Library) -> does it cost 4 mana or less "No"
Remove the top card from your library from the game (0 card left in Library) -> does it cost 4 mana or less "No"
Remove the top card from your library (you can't - go to next step?)
play the card for free (you can't you found no card with cost = 4 mana or less - go to next step?)
Shuffle the removed cards and place on bottom of the Library...
The effect on the stack triggers... Probabily the Cascade cards main effect.
Panic...
Just checked the rules
MTG rules wrote:5/1/2009 After you're done exiling cards and casting the last one if you chose to do so, the remaining exiled cards are randomly placed on the bottom of your library. Neither you nor any other player is allowed to know the order of those cards.
yeah you just shuffle and get on with the game...