In modern email, it's easy for plaintext and HTML parts to drift apart
I recently read When The Text And Html Disagree (via, itself via), which is about an instance where an email message had an important disagreement between the plaintext part and the HTML part. In this case it was fortunately obvious that something was wrong, but I'm sure there have been less obvious instances.
I believe that one reason this drift happens comes down to that old aphorism that if you don't test it, it's broken. For email with alternate parts, the revised aphorism can be said as “if you don't see it, it's broken”. Modern email clients normally show you the HTML part to start with, and then most make a generally rational decision to make it at least hard to see the plaintext one. So when people look at test versions (or real versions) of such email messages, only the HTML part has to look good in order for the whole thing to seem fine. The unseen text part can quietly rot away, noticed only by unusual people like me who look at the plaintext version.
(You would think that mass email authoring environments would raise an alert if you only edit the HTML portion of a standing mixed-part email, but apparently not.)
I've seen this sort of thing for spam, but When The Text And Html Disagree makes a nice illustration that it's not just spam that suffers from the issue. In the end we probably shouldn't be too surprised about any of this, because keeping multiple things in synchronization is pretty much a hard problem all over. If you want it to work reliably you need to automate it, and automating this sort of update isn't easy.
(Keeping things in sync by hand is extra work, and sooner or later extra work doesn't get done or doesn't get done right. People forget, people make mistakes, people will get to it tomorrow because there's an urgent thing right now, and so on and so forth.)
PS: Given this, the most likely answer to the question in When The Text And Html Disagree is that if there's a disagreement and it's not clear, the HTML part is right and the plaintext one is wrong. It could be that you have a rare email where someone has updated the plaintext part but not the HTML part, but the odds are very good that it's the other way around. The exception to this is if you're in a very unusual environment where most people see the plaintext part instead of the HTML part.