There are a few different scenarios that would render a will invalid. Some of these scenarios include:
Fraud: If any piece of the will (or any of the assets listed) are found to be fraudulent, then the entire will could be deemed invalid
Mental state: if the will was drafted while the person was not of sound mind, then it is not a valid will
Undue influence: if someone faced pressure or other inappropriate influence when creating the will (usually in an attempt to defraud someone of their assets) then the will is not valid
Improper execution: if the will was not created correctly and did not have the correct witnesses, it is invalid