A couple of times recently when I attempted to edit a post while reviewing it from the "First Posts" queue, I received the following message when I tried to submit the form:
This edit would have to be a suggested edit. (x)
This is a rather cryptic message, considering that edits made from a review are suggested edits, subject to the same peer review process. There was no indication of how the edit would have to be submitted differently, much less why. I searched both help and meta for more information about this message, but came up empty.
The first time it happened, after I cancelled out I saw that someone else had just edited the post, so I thought maybe that's why I couldn't submit an edit from the review (and wondered why, if that's the case, the error message didn't just say that instead of telling me something that doesn't make any sense - that the edit I'm trying to suggest can't be submitted because it would have to be a suggested edit). However, the second time I received this message (last night, for this question: Codeigniter Uploading files into gallery folder), there was no other edit. So, I clicked the No action needed button and tried to edit the post outside the review process, only to discover that the edit link was grayed out.
I see several explanations of possible causes for graying out the edit link here: Always show the edit button on questions. However, there was no tooltip when I hovered over the disabled link, and the edit queue was in the single digits.
A few minutes later, I noticed that the edit link was no longer grayed out, so I edited the post and my edit was accepted.
My best hypothesis based on my experience with the error so far and the explanations I've read here at meta is that someone submitted a suggested edit while I was reviewing the post, which (a) caused me to get the cryptic error message and (b) caused the edit link to be grayed out when I accessed the post the normal way. Then a few minutes later, the suggested edit was rejected, opening up the question for new edit suggestions.
Rather than speculating about it and trying to determine the answer scientifically, it would be helpful if someone could give me the answer (as Seinfeld fans might put it, "Why don't you just tell me why I can't edit the post?"). It would be even more helpful if the error message itself provided the explanation, or if the message were defined somewhere in the help pages. But, at the very least, hopefully there will soon be an answer on meta to enlighten people who scratch their chins over this in the future. :)