Why must uncommitted transactions be undone in backwards order?

I have a database log where some transactions win (they are committed before crash) and some lose (not committed yet). We learned in class that the losers' actions have to be undone backwards.

Is there any reason for doing this backwards? Can anyone give a simple example of a log where forward undos would give wrong results?

Original transactions:

• Insert record $r$.

• Update some field $f$ of $r$.

Forward undo:

• Delete record $r$.

• Reverse the update to $r$ - oh wait, $r$ no longer exists! This causes an error.

StackExchange Computer Science Q#60525, answer score: 36