There is no Undo when it comes to working with PDF tags, which really stinks, so you need to remember to save…save…save.
If you forget to save and or saved after a mistake, don’t panic (like I did at first and thought I had to start over). You can extract a page from the original PDF (Document >Extract Pages > fill out page(s) you want to extract) tag it properly and then insert it into the document at the proper place(Document >Insert Pages > fill out where you want to insert page(s)). After you’ve made sure it was inserted, delete the incorrectly tagged page (Document >Delete Pages > fill out page(s) you want to delete).
Sometimes when you move content reading order or even when you tag content the text disappears. This is because there is some sort of hidden element behind which the text is hiding. Sometimes by moving it in the reading order panel you can make the text visible again, but often you cannot. This is a reason you should save as you go along, otherwise you’ll have to close the file without saving and will need to re-do a lot of work, depending on the last time you saved.
Even though it makes the process take more time, it is sometimes a good idea to check for accessibility after each page is finished. I like to check page 1, then pages 1 – 2, then pages 1 – 3, etc, because sometimes the report claims that items are inaccessible but does not tell you which page they are on. If you check each page at a time, then you can narrow down where the problem is.