There are examples of generational curses in the Old Testament, for instance the case of the sons of Eli, found in I Samuel chapter 2. However, they were relatively rare. On the other hand, there are no examples of any curses being broken in the Old or New Testaments.
Exodus 20:5 and similar passages are often quoted to support the idea of generational curses, but that verse is not speaking about them. Rather, Numbers 14 illustrates that what this means is the punishment due the parents will not be withheld even though the children will suffer from the effects.
So many Christians impose this false belief system upon new believers, yet they can never point to a single example of any curse being broken in Scripture. There are examples of fulfilled curses such as the 70 year exile in Babylon, but it was not broken.
I'm in my second decade of studying and writing on this subject and the one thing that doesn't change is the emotional attachment many Christians have to this subject. They insist and affirm that generational curses are something that must be broken in our lives but can never point to any Scriptural examples. Their response to questioning is always something to the effect of:
Trust me they are real!
My experience tells me we must break these curses!
Here is what happened to me!
I believe many of these people are well meaning, However, many enjoy the attention they receive from new believers when told of these so-called powerful forces controlling their lives.
I don't want you to 'believe me.'
I don't want you to 'trust me.'
I don't want your attention.
Simply crack open your bible and look for any discussion of breaking curses. Then, look at Exodus 20:5 and the other five occurrences of that motif in Scripture--in context. That's what I did and came to conclusions far different than the honest Christians mistaken about this and different from the curse hustlers.
I have dealt with many Christians that have been very damaged from these beliefs. Those promoting this should answer the simple questions I have proposed or stop teaching what they cannot verify from Scripture. We are all going to have to give an account on how we handle God's Word. I genuinely fear mishandling His truth.