How and why your sites may break, and what to do, after applying March 2020 update to CF2018 or 2016
To be clear, I do not mean with this warning to suggest that you should NOT apply the update! It implements an important security fix.
Instead, it's that after applying it, your CF web sites served via IIS or Apache WILL likely break initially, until you take one at least and perhaps two extra steps. The good news is that these steps are both easy and documented by Adobe in the update technotes, but they do require that someone do them, if needed. Let me explain.
[Update: I did an abbreviated version of this post on the Adobe CF portal: Three reasons your sites may break, and how to fix them, after applying March 2020 update to CF2018 or 2016. Note I also titled it differently. Just trying many ways to get people's attention. That post may interest some, either to read first (but my TLDR below also tries to abbreviate things also), or especially if you may prefer to give others a link to a post on this matter that is not as "dense" as this one. :-) I do point to this post from there, of course, for the many additional details that some may appreciate.]
Sadly, because many people don't bother to read the CF update technotes (linked to below), and they just apply the CF updates, they are not noticing this issue until they or their users start screaming because their sites are down. There's also a fair bit of "screaming" in the CF community, and folks responding may not know the info that I (or Adobe) have shared, to get things "working again", so I hope this helps bring some calm, and most important the clear solution/s needed.