Removing, renaming shows break comments
This is an issue I came across earlier already but now came across details.
When you released v2, a lot of my comments simply vanished and it was said several times that comments from v1 were still being imported but no updates on it at all.
So, we're pretty much in the dark about this whether comments were really lost or never imported as stated, since there's no way to remember all your comments.
However, today I was looking at a user who send me a follower request. I tend to look at the profile as to find out why that person would want to follow me, perhaps because we watched something together that I commented on at some point or alike.
That person had a movie in the recent history that I commented on, so I clicked it to find out whether my comment got a reply or thumbs up by that person. Turns out, my comment wasn't there.
Since I was 100% sure I commented that terrible movie I've checked my comments list and see, the comment is there for that movie.
But not quite: The URL of the movie changed.
That means my comment on the movie was for another entry of the movie that got deleted or renamed. I assume that the runtime of that movie is either not counted anymore because - well - deleted towards my watchtime or is counted and an issue arises: I can't remove it anymore since it is technically not in the database anymore.
If I click the remove all plays, I get no notification at the top, trying to remove the rating, no notification either, rating stays the same, played icon still is activated. So this whole entry in my comments list is an orphan, including a play, rating and comment.
I simply copypasted the comment to the new entry because I see no other choice here.
TL;DR:
When you update your data from TVDB or delete shows manually or whatnot, it seems that comments are either not merged with another ("correct") entry or simply deleted without consideration. Either way: it needs some kind of check to prevent orphans and ensure the comments of users aren't lost in the void for no one else to see.
Entry I am talking about:
The Voices (movie), deleted entry with my orphan comment:
https://trakt.tv/movies/the-voices-2014-01-01
New entry, with my copypasted comment:
https://trakt.tv/movies/the-voices-2014
(trakt username is ds1 as well)
If something like this happened here, this could happen anywhere else, too.

Please see my first response, orphaned comments will be hidden from view moving forward.
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AdminJustin (Founder, Trakt) commented
It doesn't look like this is a common issue and I'll get a fix in place so that orphaned comments don't display under user profiles. I remember with this movie in particular there was some weirdness in that there were multiple TMDB entries which were also synced down to Trakt.
When a show or movie is renamed, that doesn't affect anything since the underlying record is still the same. If a movie or show is removed, all the associated data is also removed which is typically fine since the duplicate has no data associated with it. In cases with more data, I look into those and merge data as needed.
As for importing comments, some v1 episode comments are outstanding, but all movie and show comments were imported.