Can't install Flash - Installation failed
I had this problem on &amp; off for years, it's something to do with my firewall, which I've never been prepared to fiddle with just so I can get an online installer to function properly. 

The simplest workaround is to go get the full downloadable .dmg version instead of the online installer.

For the past few years, it's been located at http://www.adobe.com/uk/products/flashplayer/distribution3.html
or you can Google "adobe flash player offline installer download" which always finds it.

I did eventually find a solution to my Flash problems...
... I uninstalled Flash from every machine in the building ;-)