And segwit allowed for larger amounts of data overall (to make Lightning possible at a later date)

While transactions are signed, the signature does not currently cover all the data in a transaction that is hashed to create the transaction hash. Thus, while uncommon, it is possible for a node on the network to change a transaction you send in such a way that the hash is invalidated. Note that this just changes the hash; the output of the transaction remains the same and the bitcoins will go to their intended recipient.