There is a webpage I visit where I click on a link and some Javascript kicks in to display a textbox that was previously invisible.  Recently, I went back to this webpage and noted that the textbox was no longer appearing.

Knowing a bit about Javascript and Google Developer Tools, I knew to open the Console and look for any error messages.  Sure enough, I saw this:

Uncaught TypeError: Cannot read property ‘msie’ of undefined

Doing a decent amount of research, I finally came across this link that is fairly specific to WordPress:

In short (and this is not specific to WordPress), there might be some jQuery somewhere that uses an older API that is no longer supported.  To fix this, you want to include the following JS in your page:

Doing that should make errors, like the one above, disappear and prevent your webpages from crashing in the middle of Javascript execution.