I'm guessing a lot of them stem from using session variables and from caching. I've seen everything from an nHibernate cache being stored in the session, serializing and deserializing a huge chunk of the database on every request, to developers not being aware that the session variable they get is user specific (They access it via Session[UserId]).
Caching in particular seems to be something that's applied without much analysis, I've seen it slow down implementations more often than it sped them up.
What I'd really like is for the browser to send a tab id on every request, so we can scope variable to a tab as well as user.
localStorage is often used for it's persistance across closing/reopening the browser, but for times when you don't want storage to bleed across tabs or closing the browser then session storage is much better, and unlike a cookie doesn't automatically get attached to request so there aren't XSRF worries.
Caching in particular seems to be something that's applied without much analysis, I've seen it slow down implementations more often than it sped them up.
What I'd really like is for the browser to send a tab id on every request, so we can scope variable to a tab as well as user.