Heureka - from the -Ofun department
Or - running the ackermann function (and possibly other recursive functions) really fast.
$ time ./parrot -Oc -C ack.pir 11
Ack(3, 11) = 16381
real 0m0.567s
user 0m0.559s
sys 0m0.008s
$ time ./ack 11
Ack(3,11): 16381
real 0m0.980s
user 0m0.978s
sys 0m0.002s
The latter is optimized C code. Parrot is recompiling (or will do when that's finished) statically known and simple PIR code to native machine code via it's JIT system.
See also a recent post on perl6-internals.