Just another update on my Error 80 experiences with the 5D mark III. I'm still getting "Error 80" messages, approximately once per day on average (although yesterday I got 3-4 errors over the course of maybe 30 shots). I've had this error 25+ times or so over the course of maybe 1,500-2,000 pictures.
I'm still using the Eye-Fi Pro X2 (8GB) SD card along with a CF card. (The Eye-Fi capability is VERY important to me, and I don't really want to work without it.) I've used numerous CF cards and Canon lenses (and always a genuine Canon battery), and the problem seems to happen without regard to lens or CF card being used. I'm pretty sure it's related to the Eye-Fi card.
Also, I have noticed that the error 80 definitely happens (at least sometimes) when I press the shutter button. (I use "silent continuous" mode. I might be imagining things, but the shutter seems to sound slightly different when I get the Error 80 message, and the picture definitely does NOT get recorded.) I think (but am not 100% positive) that the Error 80 also happens sometimes when the camera is just sitting "idle" (powered "on" but not being used), because I have picked the camera up to take a picture on several occasions only to see ERR 80 on the top LCD...
I'm hoping this can be fixed via some sort of firmware update. I have checked my battery (and CF and SD cards) and battery compartment for any signs of dust/dirt/residue, and they all look clean and brand new. I have never used my 5Diii in a dusty/dirty/sandy/wet/humid location, so I am very skeptical that it has anything to do with the contacts on the battery or anything like that.
Does anyone have insight as to whether these Error 80 problems are limited to Eye-Fi Pro X2 cards, or are they also occurring with other versions of the Eye-Fi cards? I think I saw something about Canon admitting a problem with the 60D (?) and Eye-Fi X2 cards - maybe the 5Diii has a similar problem....?
Just a note/disclaimer - while I am unhappy and annoyed with this Error 80 problem, I am thrilled with the 5Diii otherwise. I have used the 5D "classic" and 5Dii since they came out, and this camera is a major improvement IMO. My 5Diii can nail focus with the 85L wide open using the outer focus points -- something I couldn't do consistently when using the 85L for more than 5 years with the previous versions of the 5D series. Also, my camera is in the serial number range affected by the "light leak problem," but I have not noticed any sort of problem at all (and I generally use the camera in low-light - ISO 3200 and 6400 with apertures in the f/1.2 - f/2.8 range and shutter speeds around 1/100 - 1/250).
Thanks!