If you make a mistake get some seeds into your inventory and use the Adventure>landscaping plant grass to turn it into grass tiles. You can then plow it again.
If you pave where crops are growing they will keep growing. They can still be harvested when they are tall. But then you cannot plant there unless you put grass and plow.
Paved land will decay back to the tile under it after a long time.It takes much longer than plowed land. Many people pave so that they can run fast and so they do not see weeds.