Plans in general fail as soon as they hit cold reality. There will be problems with implementation, inglorious watered-down compromises, corruption, unforeseen consequences, etc. The funny thing is though that political radicals will not view their original plan as flawed, only the 'deviations' from that plan.

For example, some communists would argue that the reason communism failed was because it deviated from the "paper" version. See 'actually existing communism' and the Nomenklatura (New Class) for more information.