These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Steamboat Willie
Common Knowledge: As noted on the main page, this was not the first appearance of Mickey (or Pete, for that matter). It is also a very common mistake in animation history that this short was the first sound cartoon, when those existed as early as 1924 (and Paul Terry had taken a shot at the sound cartoon called Dinnertime just a month before Willie was released).
Older Than They Think: As attested on the main page and above, Steamboat Willie was not even close to being the first sound cartoon, which Max Fleischer had been doing as early as 1924 with his Song-Cartunes series, and Paul Terry had created his own sound film, "Dinnertime", just a month before Willie was released.