When editing a post, that edit will be combined with the previous one, or the original post if there was no previous edit, but only if not too much time elapsed between these two. It would be nice to have a clear indication while editing whether that edit will be combined or not. Particularly when writing the edit summary, it is important to know whether to describe just the last bit, or all parts of it.
I know that if the edits will be combined, then the original edit summary will again be included in the summary field, to be modified or not as appropriate. But if the original edit didn't have a summary, or if there was no previous edit, then this hint is too weak. And even with that hint, it took me a while to figure out that replacing that summary to only describe my latest modification might be a bad idea.
So I'd ask for a stringer indication as to whether the edit will be combined or not. Nothing cryptic, but a clear line of understandable text, or something equally obvious.