Let's take Disney parks from before around 1995, since that is what a lot of people think was the time where the parks started going downhill.
Yes, we know that in general things were very well done, but there was still some mis-steps before.
Here's mine:
The Christmas star on the Matterhorn in Disneyland- Still surprised that Walt approved this one...it really messes up the scale of the Matterhorn, similar to what happened when Epcot had that wand years later.
The Barbie show at EPCOT Center (1994)- No explanation necessary.
http://waltdatedworld.bravepages.com/id134.htm
Making the Star Tours exterior at MGM Studios/DHS so small and cramped. I think they had a great opportunity to create a real mini-land back in the days when they built Star Tours, but they instead just put a few different planets of scenery in one place. Yes, it's "Studio" themed (aka, can be done cheaper and less themed), but I always thought it was a missed opportunity.
Giving up on building out the World Showcase- EPCOT Center went through lots of changes and updates over the years, but the World Showcase had only very few. Two extra pavilions in Norway and Morocco, but no other attractions or new countries and only some overlays or updated films in this decade.
Not building POTC at WDW when the park opened, then building an abbreviated version after.
Yes, we know that in general things were very well done, but there was still some mis-steps before.
Here's mine:
The Christmas star on the Matterhorn in Disneyland- Still surprised that Walt approved this one...it really messes up the scale of the Matterhorn, similar to what happened when Epcot had that wand years later.

The Barbie show at EPCOT Center (1994)- No explanation necessary.
http://waltdatedworld.bravepages.com/id134.htm
Making the Star Tours exterior at MGM Studios/DHS so small and cramped. I think they had a great opportunity to create a real mini-land back in the days when they built Star Tours, but they instead just put a few different planets of scenery in one place. Yes, it's "Studio" themed (aka, can be done cheaper and less themed), but I always thought it was a missed opportunity.
Giving up on building out the World Showcase- EPCOT Center went through lots of changes and updates over the years, but the World Showcase had only very few. Two extra pavilions in Norway and Morocco, but no other attractions or new countries and only some overlays or updated films in this decade.
Not building POTC at WDW when the park opened, then building an abbreviated version after.