There is an easy imaginable scenario for an bitcoin fail:

1. The hash rate drops to a point, where a 51%-attack is possible. This can be due to very low price and/or MtGox going offline (see MtGox-hack events earlier this year). What ever the reason - with a possible 51% attack bitcoin gets a problem ...

2. Businesses that ARE using bitcoins today get cold feet and bailing out - dropping price and hashrate even more.

3. IF an successfully attack takes place, most of the remaining businesses have to bail out -they can't take the risk. This would destroy the demand ...

4. A recovery from this loss of faith (security) would be difficult at best ...

If you want a better idea of possible fail scenarios, have a look at the alt-chains. None of them has actually failed - but more than one has been called a "zombie" ...
Dont't get me wrong - I like alt-chains (and mine them like hell with the i0coin- and iXcoin-resurrection), but they provide interesting "what-if" scenarios for failing of a chain ...