CounterSpell

In a fit of pedantry, Kazuhiko points out that there is nothing saying the die can not have '6' on every side.

Hehehe.  Nice point.  But "Roll a six-sided die" is the standard wording for all Magic cards that use the ability - MTG: Goblin Tutor, MTG: Urza's Science Fair Project, etc.  I guess they're all equally exploitable :)  This is when your opponent sideboards in their MTG: Goblin Bookies, I guess... --AlexChurchill

Feeling truly pedantic, I went and found the official ruling.  And it's bugged!

Which does nothing to prevent you from having a die marked 6,7,8,9,A,toothycat.  --Vitenka  Or, as has actually happened; Arrow, Blank, Cross, Explosion, Skull, SquigglyThing?.  (We rerolled.)

In this case, nothing to stop you doing so, but not much to benefit.  However, with something like MTG: Spark Fiend, using dice with the values 10,20,30,40,50,60 would be quite an advantage, and there's nothing in the Unglued rulings covering those cases.  But sooner or later, you have to appeal to some degree of common sense on the part of the players... --AlexChurchill

The only really sad thing about this is that StuartFraser knew this ruling without having to look it up, and was wondering how long it would take before somebody quoted it.