Users with select only privs are able to modify data in the data grid. Although an error is received and the actual data is not changed on the database, Toad places a ROW SHARE lock on the table row. This causes unnecessary locks for other sessions that is trying to update that particular table.
Ensure users close Schema Browser or put Toad's data grids into read only mode from View | Options | Data Grids - Data and check "Use read-only queries"
To avoid any possible editing for a particular connection, the 'Read-only' flag on the connection itself can be set.
Press the rollback icon from the data grid.