I tried hard resetting my LG Optimus F6 / D500 after being stuck with a "Security Error" on boot and not being able to use my phone. I first tried accessing recovery mode, but gave up on that (The only thing I am loosing is my contacts and I have ways to obtain the ones I care about.).
(It turns out that I made my way to the hard reset differently than I read I was supposed to online. I think what I did was this: I had the USB plugged in and after popping the battery out and putting it back in I got the phone to turn on. It went through a couple of cycles of displaying the error before I tried this exact sequence: I let it get to the security error and then once the screen went black I held the volume down and power buttons as I watched the LG logo appear and then disappear and at that moment I released.)
I currently have my phone plugged into the USB cord to my computer and after confirming that I really do want to go forward with the factory reset it gave me this message:
 Secure booting Error!
 Cause : boot certification verify
It is still on in this state. I recently started to hold buttons, but nothing happened. I have made no further action.
Also, before booting I had removed my SD and SIM cards just in case that would help.
This phone has never been rooted, but it's past its warranty. I think that some of the issues is due to the number of times I maxed out on the internal storage and cleaned up space, including caches to force the currently desired activity to function (1 GB internal is not great) without ever taking the time to back up and reset my phone to clear up any corruption that may have occurred along the way. Aside from that, I really don't know what I'm doing. I mostly just figure that I'll consider my phone bricked and hopefully some useful components can be pulled out by a technician who is willing to spend his time fixing other F6s. This phone has been through it: I even rolled over it with the front tire of my car in a parking lot. It still worked after that, I think because it was directly under the tire and face down, along with being inside a rugged case.
After telling my life story (I spared you the details of where I got the phone), my question is simply this: Even though I will be getting a new phone (finally), what might I do from here to successfully reset the phone and get it working again so that someone (with very low expectations) could potentially use it for themselves? The biggest reason I still care is that I want to leave a trail for others with the same issue... should they exist.