The data availability problem
The problem with data availability occurs when the transaction data for a newly proposed block cannot be downloaded and verified. This type of attack by a block producer is called a data withholding attack, which sees the block producer withhold transaction data of a new block.
Since transaction data is withheld, nodes cannot update to the latest state. Such an attack can have numerous consequences, from halting a chain to gaining the ability to steal funds. The severity of the consequences will depend on the type of blockchain (L1 or L2) and whether data availability is kept on-chain or off-chain. The data availability problem commonly arises around L2 scaling solutions like rollups and validiums.
Read more about the data availability problem.