Can be about an individual cartoon film, a series of shorts, or even an early TV series.
For TV airings or cinema rereleases some films just had to get the title cards (or even the animation itself) altered in someway or another, and some good title art designs or important credits just went away never to be found.
What are the most annoying alterations you've seen on these cartoons?

Not to make a poll since the answers will vary quite largely...

Any NTA-retitled Paramount cartoon in general, but especially any cartoon with a pan or a truck over the titles.

The aap retitled Technicolor Popeyes, for the horrors they did to the soundtracks. Were still feeling the fallout of that.

Probably the CBS TV Prints of Terrytoons where they cut out half a minute in most shorts.

It's hard to top the butchery the Terry-Toons suffered from CBS, but when it comes to title card alterations, I have to hand it to Samba Pictures and their handling of the B&W Mintz/Columbia cartoons (Krazy Kat, Scrappy) for their carelessness as far as accuracy of the credits on their cards to the original credits. The Mintz cartoons up well into 1933 gave the directors a "proper" credit ("A Charles Mintz Production (or early on, "A Winkler Picture") by [...] ), but all of the Samba titles use a "template" form with one each "Story" and "Music" slots and three "Animation" slots, and the credits were "adapted" to this form (the real credits only changed to the Story/Animation form in 1933, right around the time most of the directors walked out on strike - perhaps not a coincidence), sometimes losing names credited on the original release and sometimes even adding names not credited. A few Krazy Kats that were solo-directed by Ben Harrison for the 1931-32 season have TV credits so wrong that Joe de Nat's credit is the only correct one. Even some of the TV credits for post-'33 cartoons are off the mark, in part because of the limitations of their TV title "templates". On top of this, a handful of cartoons also had footage deemed too risque edited out of the 16mm TV negs (although I wonder if Scrappy's Showing Off had the original ending cut from the 35mm source).

It's bad enough when TV titles drop credits altogether, but outright erroneous credits are on another level because they mislead people.

While the "black bars" on NTA's color titles are unsightly, I say they're underrated, because the earlier efforts by UM&M are worse, completely replacing the original title sequence with bland yellow lettering on a red background. Note that Suddenly It's Spring has completely fake titles on the Thunderbean Noveltoons DVD, because it's from a UM&M element. At least the "black bars" NTA treatment lets us see most of the original titles, albeit in "censored" form. Note that NTA made some new TV negatives later on (various late-'40s Screen Songs being good examples) where they only replaced the Paramount logos and didn't bother hiding any other Paramount or color-process references.

How about the erroneous credits on the reissue titles for Mail Dog and Morris the Midget Moose? For example, the D23 website says that Mail Dog was directed by Charles Nichols  and that Morris was directed by Jack Hannah , but the titles we usually see suggest vice versa...

I suppose with all of Samba Pictures' carelessness there is one positive thing worth mentioning. While they did cut the TV titles into the original 35mm negatives in at least many cases, apparently they just rolled up the original titles and stuck them back in the can with the neg, based on what has been found with many of them in recent times. UM&M could've been so thoughtful. (Heck, so could the studios have been when preparing theatrical reissues... though it sounds like Columbia did the same as Samba in at least some cases.)

I also should've mentioned the 1931-32 Krazy Kat season instead of 1932-33. Relying on memory can fail you sometimes...

The older Blue Ribbon shorts. A lot of shorts from this batch don't have their original titles existing (leaving them unknown), but I know for a fact "Old Glory"'s is easily predictable (the DVD version with the Blue Ribbon title and original ending actually flows well)...

And also, "Book Review" makes no sense when compared to the original title's pun.

It's hard to top the butchery the Terry-Toons suffered from CBS, but when it comes to title card alterations, I have to hand it to Samba Pictures and their handling of the B&W Mintz/Columbia cartoons (Krazy Kat, Scrappy) for their carelessness as far as accuracy of the credits on their cards to the original credits. The Mintz cartoons up well into 1933 gave the directors a "proper" credit ("A Charles Mintz Production (or early on, "A Winkler Picture") by [...] ), but all of the Samba titles use a "template" form with one each "Story" and "Music" slots and three "Animation" slots, and the credits were "adapted" to this form (the real credits only changed to the Story/Animation form in 1933, right around the time most of the directors walked out on strike - perhaps not a coincidence), sometimes losing names credited on the original release and sometimes even adding names not credited. A few Krazy Kats that were solo-directed by Ben Harrison for the 1931-32 season have TV credits so wrong that Joe de Nat's credit is the only correct one. Even some of the TV credits for post-'33 cartoons are off the mark, in part because of the limitations of their TV title "templates". On top of this, a handful of cartoons also had footage deemed too risque edited out of the 16mm TV negs (although I wonder if Scrappy's Showing Off had the original ending cut from the 35mm source).

It's bad enough when TV titles drop credits altogether, but outright erroneous credits are on another level because they mislead people.

While the "black bars" on NTA's color titles are unsightly, I say they're underrated, because the earlier efforts by UM&M are worse, completely replacing the original title sequence with bland yellow lettering on a red background. Note that Suddenly It's Spring has completely fake titles on the Thunderbean Noveltoons DVD, because it's from a UM&M element. At least the "black bars" NTA treatment lets us see most of the original titles, albeit in "censored" form. Note that NTA made some new TV negatives later on (various late-'40s Screen Songs being good examples) where they only replaced the Paramount logos and didn't bother hiding any other Paramount or color-process references.

I have some of those Screen Songs on various public domain releases. I wonder when they were done -- those are the ones with the NTA "map" intro, right? It seems clear that NTA retained elements with original titles in order to do this.

I have seen some where they re-shot the final card showing the title of the cartoon, in a different lettering style from the original, with a UM&M copyright line sans black bars. "Winter Draws On" is one example of this, with a completely replaced final title card; meanwhile, the same cartoon was on a Republic VHS tape with original titles (apart from the NTA "map" logo).

There are some Betty Boop cartoons which show up in different places, sometimes with Paramount's name left in the copyright line, sometimes with a refilmed UM&M copyright line. They must have had multiple negatives for these. "Betty Boop's Crazy Inventions" and "Minnie The Moocher" are two examples of this.

UCLA has all the original negatives, right?

Yes, I believe UCLA has the negatives.