Yep, I mean internet browser cache. Just made a mental typo.
Granted, I was only repeating what I saw on some of the sites. I did
a google search with the terms "hotmail include original message in
reply" and browsed a few of the responses since I wasn't able to
replicate it on my hotmail account (I don't actually use that account
anymore other than a junk email for registration on various sites).

The following link appears to be the related bug report for the
behavior you describe:
https://bugzilla.mozilla.org/show_bug.cgi?id=538892