Rant: spent 6 hours on a PC that just needed a new CMOS battery
Client's old Optiplex wouldn't keep time or boot right and I tore apart the whole thing, reseated RAM, swapped the PSU, ran every diagnostic I had before my coworker walked by and asked if I checked the $2 coin cell in the back.
The "$2 coin cell in the back" line got me, because that is exactly how these things go. I read a thread on a repair forum a while back where a guy went through the same thing with a machine that kept losing its clock and he replaced half the board before someone mentioned the battery. The part that stuck with me was a comment saying those cells often test fine on a meter but still fail under load, so people skip right past them. Six hours is rough but at least you had a coworker around to say something. I keep a spare in my drawer now just so I stop making that mistake.