I recently posted a very intriguing and irritating problem with a C++ program on the GCC help group. A certain piece of (incorrect) code was returning a defined answer when compiled with DevC++ and g++ on linux. Have a look at the problem and the solution here.
If you are really interested in competing with the best at the highest level, come for Cognizance, IITR's tech fest. Plenty of programming opportunities (multiple programming events and great cash prizes). Make sure you check out the E&C department's and Maths Department's list of events.