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.