I've got a Late-2011 15" MacBook Pro stuck in the gray screen when booting AFTER login. I've tried removing the drive (it's non-retina) and booting from a working bootable Yosemite installer, no luck. (it sees the startup USB drive, the progress bar progresses, but stuck at the gray screen again even without a drive.
Tried NVRAM reset (cmd option P R), it has reset (the login sound's volume changed) but nothing changed.
Tried holding D on startup, AHT doesn't start, instead, it tries to boot into the OS.
Tried Cmd V, it sometimes works and displays the verbose output, sometimes, it acts like I haven't even pressed. Again, stuck after login.
Tried booting again from USB, I select USB (after seeing both drives correctly), but no luck, it's also stuck.
I've stumbled upon this:
Boot hangs on grey screen (even when booting from USB drive with fresh OS X install)
I've tried booting into single user mode with Cmd S, it worked ONCE, I've disabled the graphics extensions just as shown at that post, restarted, but again, nothing changed.
More weirdly, now the Cmd S doesn't work either. It just takes me to login screen without showing the terminal, just as I haven't pressed anything.
Why would this happen? Why would a Mac refuse to enter hardware test mode, single user mode, or refuse to boot from a working bootable Yosemite drive both with or without the main drive installed?
UPDATE: I've run the AHT and got an error:
It looks like a memory issue, but I'm not sure.
UPDATE 2: I've tried reinserting the RAMs and the the problem is not reported anymore. However, the Mac still won't boot.