slippery slope

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A tricky precarious situation, especially one that leads gradually but inexorably to disaster.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic A chain of events that, once initiated, cannot be halted; especially one in which the final outcome is undesirable or precarious.
• noun An argument that follows a chain of events or causes and effects to some conclusion.