On a Mac mini end 2009, with a drive updated to a SSD, and RAM updated to 8GB running OS X 10.11.6, I sometimes get a "🚫" on boot. It then boots in Recovery mode by itself, so i shut it down and power it up again, and it boots normally, without any problems after.
Maybe it's not the right place to ask this, I'm a StackExchange noob.
But how does this happen ?