Catastrophic bugs
The upside of an immutable ledger is that nothing can be hidden or altered. The downside of an immutable ledger is that nothing can be hidden or altered—including bugs. In the case where networks such as Ethereum write the smart contract code into the chain itself, this means that code bugs cannot be fixed easily; the original code will remain on the blockchain forever. The only workaround is the modular code where each section references some other section, and these pointers have programmatic ways of being updated. This allows the authors of a DApp to upload a new piece of code and adjust the pointers appropriately. However, this method too has issues. Making these updates requires a specific authority to update the code.
Having a central authority that is necessary for updates just creates new problems. Either that authority has centralized control over the decentralized app (which means it is no longer decentralized) or a governing system must be written onto the blockchain...
