It's not that hard to come up with scenarios.

1. Accidentally deleting an old transaction

2. Accidentally modifying the value of an old transaction (and not realizing which transaction)

3. Accidentally overwriting the payee/notes of the transaction.

The list could get very long, but basically: "Accidentally" anything. I think it's even more likely if you are dealing with downloaded transactions.

It's happened a number of times that I am entering perhaps 10 transactions into it, and I screw something up. My only option is to quit and restart everything.

Those scenarios sound better served by version control - or at least an undo that has version control behind it.

In any decent accounting system, you cannot accidentally modify anything after it's written. It's a log system, you can only append.

If gnucash doesn't do that, I'd consider it disqualified for any kind of accounting.

Not an accountant, but that would suck more than not having undo.

Real accountants may be very diligent. The average user (even tech user) is going to make tons of mistakes while entering data. I've often found when reconciling accounts that I put a transaction in the wrong account and I fix it.

You can still do that. It's just a move from one account to the other. "Undo" is booking the transaction in reverse.

It's fine if that's automated, the point is that it's recorded in the first place.

(OK, I misspent my youth as an assistant to an accountant. With actual hand-written ledger books. Maybe I am super-picky about these things :)

this is true of GnuCash, but only after a transaction is marked as reconciled

You're correct. A general ledger does not have an 'undo' function, it would be un-auditable!