The following just occurred to me: there was a suggested edit, I saw a couple of other things that could be improved so clicked "Improve". While I was busy improving, I got a notification that "This post has been edited while you were editing" (or whatever the wording is). Looking at the post, it appears that what happened is that someone else approved the original suggested edit.
I would consider that notification to be the wrong way around. I would have thought it more suitable for the second person to have received a notification that someone else was already considering the suggestion and that they should wait a few minutes to let them finish. I only got the notification when I was almost finished, but clearly couldn't then submit my edit. And once I'd gone back to the question and seen that all that had happened was the original suggestion being approved, I didn't really feel like going back and re-editing with my improvements.
I wouldn't make this a hard lock. Just a message, "Someone started improving this edit X minutes ago. Do you really want to continue?" would suffice, I deem.
Added in edit: Hendrik Vogt has pointed out that the current notification should still be given if the edit is approved while I'm improving it since, under my proposal, that could still happen (or the original author might edit it themselves, for example). So my statement above "the wrong way round" isn't quite right. I don't want to replace that notification, but add another one.