However then we discovered some bugs that were in code that hadn't changed in a long time. For some reason, certain chunks of our view templates were just disappearing. Nobody could figure out why.
This is where Vim came unexpectedly to the rescue. You can write comments in erb by writing something like this:
<%# this is a comment %>You can also go like this:
<% # this is a comment %>However with the latter type of comment, it would always mess up the syntax highlighting in Vim. It wouldn't recognize that the %> was closing the erb comment tag, because it thought it was part of the comment. I had noticed this before but didn't really care that much, because it wasn't my code and I didn't really want to mess with it. But this time in order to find the bug I figured I should have correct syntax highlighting.
Anyway after fixing the comments, I couldn't find anything wrong with the file. I was still baffled. So I figured I'd just go and look at the page in the browser again, and boom! It worked!
My hypothesis is that Ruby and Vim probably use the same logic for parsing those erb comments, but JRuby uses something different. JRuby would behave exactly the same if we had <%# or <% #. Ruby on the other hand will think that anything after the # is part of a comment, and therefore will ignore the %> that is on the same line.