4D v11 SQL features support for nested transactions (see this Tech Tip). However, when converting a database to 4D v11 SQL it is important to ensure there are no unintentional nested transactions.
It is important to ensure that all START TRANSACTION calls have a corresponding VALIDATE TRANSACTION or CANCEL TRANSACTION call and ensure that START TRANSACTION is not accidentally called more than once to avoid any side effects.
In a previous version you might have written some code that calls START TRANSACTION more than once. In versions prior to 4D v11 SQL this has no effect. If you upgrade the database, the code will still have no effect because the ìAllow Nested Transactionsî check box will be unchecked in an upgraded database. However, as soon as you check that check box, your code might work differently.