Undetected Bad Transaction

[mrns]

I have spent a few hours going through the documentation and I now understand the basics behind bitcoin. Here is my newbie question: Considering that each transaction contains a hash of the previous, thus creating a chain which would require an exponential amount of work to recreate, I wonder what would happen if a bad transaction is undetected for enough time to be impossible to recreate the chain without the bad transaction.

[Gabi]

Define "bad transaction" please. It seems you are speaking about blocks. How can a block be bad? And how that would be "undetected"?