I just edited a question, changing both title and body text. It definitely a useful edit. But as soon as I completed the edit I realized/remembered there was a tag I should have added. So I looked for the retag button so I wouldn't have to send a second edit past a moderator for approval. To my surprise there was no retag button. It seems that the retag button goes away when you have a pending edit on the question. There was an edit button and I decided to go ahead and use it to add the tag. (I figured the moderator will have just looked at my previous edit so it probably wouldn't be too much of an added hassle to look at the second one.) But that left me wondering: When I saw that a tag needed to be added, should I have just let it go? Is there another approach that I could have used to fix my previous edit?
Yes, this is by design, you did the right thing.
As soon as a pending edit is in situ we disable edit and retag functionality. This helps us avoid complex merges.
However as the owner of the pending edit, you may further edit it if you remember you missed something, so just click the edit button and add the missing tag.