Here's one: Joe Johnston could have defended Snake Creek Gap before McPherson marched through it unopposed. Instead, no historian can tell you for sure if Johnston even knew Snake Creek Gap even existed or if he had given any thought to defending it.
That would certainly qualify as a missed opportunity, and an example of where Johnston could have prevented Sherman turning his position given hindsight.
What I have to wonder, though, is whether preventing an enemy advance would be counted as "something big"; it's a stalemate, which is good (assuming that Sherman doesn't instead turn the position to the right) but historically people don't always seem to consider stalemates as a positive.
As to the question of why Johnston didn't defend the Gap, I've been checking in the ORs and I have found no mention of the Gap (by either side) in correspondence before the Atlanta campaign starts. I have, however, found this:
Now, according to the information I have on positions at the start of the campaign, Wheeler had left Grigsby in that valley (west of Baylor Mill Gap, near Chattagatta Church). Hooker's 20th Corps would have had a clear but long run to Snake Creek Gap, but 20th Corps wasn't the one that went there so perhaps there was a logistical reason (a reason which would explain why the gap from The Pocket over Horn Mountain wasn't practical).
My suspicion is thus that Johnston ascribed "watch out for the enemy" to Wheeler - in his mind, if not in actual orders - and possibly considered Snake Creek Gap not within Union logistics capabilities for a significant force.
Note though that this is his possible reasoning; it is not saying he was right to do so.
Tracking what happened to Grigsby, Cleburne mentions him fighting at one of the more northerly gaps on the 8th, but says he was not there by 1AM on the 10th (and that he had been sent to Snake Creek Gap), probably in response to a report mentioned as coming in on the 9th.
Wheeler's report says he was picketing from Ship's Gap to the Connesauga before the campaign opened (which includes the viable approach routes to Snake Creek Gap, though he doesn't mention it); after that he focuses in pretty heavily on the "fun stuff" his command was doing.
A brigade at Resaca was Cantey, who arrived there on the 7th. No report from him, and probably a bit late anyway to block the gap; before then it seems to have been just a few regiments.
So it looks like a possible explanation is that:
- JEJ thought he'd have a warning if the enemy was feeling for his left, or did not know about Snake Creek Gap.
- He, Wheeler, or both missed the loss in scouting information that came in when they pulled back to the Rocky Face.
- Johnston also didn't adequately garrison Resaca if it was in any sense considered to be vulnerable beyond a cavalry raid.
The possible end result of doing this would have been a brigade or so dug in at Snake Creek Gap, and Sherman having to commit to either head-on fighting in the Rocky Face-Coahulla space or a movement turning the Confederate right.