Or hitting alt-left-arrow; Redrose's comment sums this up better than I could.
Searching for something, then hitting backspace/alt-left-arrow will, instead of bringing the previous page up, throw the page for the search index up at
https://en.wikipedia.org/w/index.php?title=Special%3ASearch&search=
Version: wmf-deployment
Severity: normal
OS: Windows 7
URL: https://en.wikipedia.org/w/index.php?diff=prev&oldid=587033474