Not a direct answer to your situation but to bring some awareness to others who are new to this less talked about topic of memory card which we sometime take for granted.
1) The remaining frames available info from the card is estimated on the camera. The remaining frames can differ when put into another camera. It's due to choice of format like RAW, Jpeg Hi, Jpeg Lo eg, scene you shot and of course the megapixel of the camera. I think that's obvious to most right off the bat.
But that figure is just an estimation. take Turbonetics card showing 503 frames left as example. I am certain by the time he max out the card, it could show 480 or 520 shots because each frame shot is not exactly the same byte size. One with less color in the scene ( a white wall scene ) could end up taking 4.6mb (jpeg) and then you shot another with lots of colors and fine details details like a tree, that file size might be 6.1mb. So collectively at the end you could have shot more or less then 503 frames.
2) I usually don't share memory cards between my D300 and D4
in between shooting. There are compatibility issues not only between camera brands but also models at time. That means what's shot on one camera (D700) might not be previewed when put into another camera like say D3. You may see the remaining frame info (shot in the D700) but you can not preview the Images or Video. Best to check if you have old and newer cameras in your collection and see if you do have this issue. Not sure about Canon. Seem like a small issue but it can bad in some situation.
3) When the tiny BYTE switches of the memory chip nearing it's ending life span, the card's "wear leveling" ability can mask the detection of a faulty or worn out bit. Unless you constantly max the card, you hardly will notice it and think it is a glitch of some sort. And seem to have gone away after a format. This single or few thousand bit that has gone bad may
show itself as some weird anomaly image like half your photo but parts of it get distorted, color run, weird artifact ..etc. This may happen more and more progressively and noticeable if you often max the card. Another system could be a bad read when you try to preview the shot or camera seem to hang while reading the card or after taking a shot.
4) Sometime those individual tiny BYTE switches can be temporary and with certain retrieval software or formatting you still might "unstuck: it. I have had some failed cards before due to wear and tear and one time possibly due to a sudden death and the card just stop working. But the worn out cards I have lasted me about 2 years plus. I always shoot to the max easily given they were 1 or 2 gig cards. I have 1 16gig cards that's coming to 3 years old and still fine but I am cautious. Over time all cards will wear out or it can just fail so take as much caution over them as you do your camera and lens.
Not to get anymore technical, Sorry if I did. Best to always mark your cards with a date on them and mark them if they have ever gave you any problem. Overtime this alert you that the card had a hiccup before and if this occur on the same card too often, it might be time to change card. I think most will not worry but for mission critical projects like Wedding, launches..etc. That is a nightmare.
Don't take them for granted.
