Requested feature overview description.
Basically, what I am thinking is that there should be a way to see what tags were initially inserted by a person into a post, before the software alias kicked in. Note that aliases would remain working the same as usual, just that you'd be able to see what the actual initial tag added to a post was, in some manner. [Would it be a Privileged-and-up feature/tool, or would normal members also be able to use it, if implemented?]

Why would it be useful?
This way, you can more easily sort out posts with invalid tags, because you will get to see where a person messed up on tagging, and can insert the correct tags. The aliasing usually becomes a problem with invalid_tag and invalid_color, and all other tagging mishaps can often be solved as-is (though not always). I've had times where I can't discern if the person tried to tag fur or skin with simply a colour and forgot or didn't realise that spaces are for separating whole tags and not words in a tag, or just used a colour because they thought it was a valid tag. Example: There is a tag for blue eyes. Okay. What is it? Why, blue_eyes, of course! Now, if a person unfamiliar with the underscore tagged blue eyes instead, it would return both invalid_tag and invalid_color. It might seem like not such a big deal, but sometimes these pieces of details can be hard to determine in a post.

The number of posts with an invalid_tag and/or invalid_color creeps up slowly every day, and it'd be cool if we could fix those images' tags in addition to keeping the invalid tags at bay.

If it were advertised somehow in the guides of the site, it'd perhaps also help newbies understand where they went wrong in tagging, and why some of their tags disappeared and were replaced with this one scary purple tag which tells them their tags aren't valid.

What part(s) of the site page(s) are affected?
Tag history pages, where it would be most useful. Also, searching for the aliased tag currently won't show it on the sidebar, only showing the destination tag. For example: searching adorable, crap, or namco returns the same results, because they are all aliased away to invalid_tag. This is normal and expected, and I, at surface level, have no problem with this. However, it can be difficult to determine the intent of the tagger sometimes, and to know whether they inserted a subjective tag, or simply one that was deemed no longer needed, or was an odd choice of tag, such as "in" (currently not invalidated itself, and I can perhaps see why, considering the mistake described in the "why useful" section).

The sidebar could also perhaps show how many posts used what tags, and clicking on them would show only those posts and not simply all invalid_tag or invalid_color posts. Searching invalid_tag and invalid_color would be the only way to display all posts with an invalid tag, but it would still be helpful, considering you could simply click that tag and be returned to the entire results. That might be crossing over into a different feature entirely, but hey, it's still relevant to the issue of invalid tags.

Great idea

+1

If you are uploading:
https://e621.net/post/upload?use_new=true -> Preview Final Tags

Because current system relies on tag aliasing, the data that what the tag was before system changed it into invalid_tag is nowhere. So this would either need to:
- Alias all tags into their own invalid tags e.g. bad -> bad_(invalid).
- Build a whole new system which would keep a track of tags which were applied to post before the system changes the tag with alias system.
- Build a system that keeps tags in memory when you uploads stuff and does comparison of the list if it see that invalid* was inserted on post.

All of these solutions would be hard to implement or create to my understanding.

Mairo said:
If you are uploading:
https://e621.net/post/upload?use_new=true -> Preview Final Tags

Because current system relies on tag aliasing, the data that what the tag was before system changed it into invalid_tag is nowhere. So this would either need to:
- Alias all tags into their own invalid tags e.g. bad -> bad_(invalid).
- Build a whole new system which would keep a track of tags which were applied to post before the system changes the tag with alias system.
- Build a system that keeps tags in memory when you uploads stuff and does comparison of the list if it see that invalid* was inserted on post.

All of these solutions would be hard to implement or create to my understanding.

It could be useful if this new uploading page would be more propagated to the users, maybe with a big ol' banner on the old page that's accessible by default. It looks really great and I didn't knew about it, probably like the rest of the userbase.

+1, could definitely be helpful for newbies

Somewhat related, but I was always a fan of a similar proposed system that shows what tags imply which, like collapsible areas showing connections between implications (and maybe aliases), and feel that would be helpful to everyone. I could see that concept merge with this one quite well, too

DiceLovesBeingBlown said:
Somewhat related, but I was always a fan of a similar proposed system that shows what tags imply which, like collapsible areas showing connections between implications (and maybe aliases), and feel that would be helpful to everyone. I could see that concept merge with this one quite well, too

I'm up for that being some sort of feature maybe using a similar code to the branching reblogs beta feature that tumblr had (when tumblr labs was enabled). It would be a very useful tool to learn new tags that you can use by traversing the branches of tags, like the many different branches off of things like penetration, cum, mammal

As for the invalid tag fixup, the only thing I could think that the code would allow in its current state is some sort of "preview tags" button that might highlight or list tags that trigger an implication or alias, though the first step would be just a preview button next to the save button on a submission.

I can personally see use if we had a preview button when editing tags, so that if I forget the wording order on a less-common tag, I can preview it to see if it comes up as a 1-result tag, without wasting my limited number of tag-changes-per-hour to click save, only to find out I made an error and have to edit it a second time to fix the error.