Thank you pflower and Geoff for your replies this is interesting information.
Curiously I had a corrupt image yesterday which surprised me since the jpeg view was fine but clearly the raw file was corrupted. It looked fine in the thumbnail window of SPP and I batch processed all of the files to Tiffs and then imported into Lightroom. This one had a bright green overlay over the entire image - you could just discern that there was an image behind it. Going back to SPP the Review window also showed the same green overlay.
This is the problem I am occasionally encountering, followed by what Geoff also reports that a sequence of images can become entangled in the corruption issue, except in my case the battery is freshly recharged.
Also last night I was bracketing whilst the battery was very low and the camera froze. The only way to address this was to take the battery out and replace it with a fresh one, which appeared to corrupt not only the images I had just taken but several images taken just before. These images were just not recorded on the card. Strange. Earlier images on both occasions were recorded perfectly.
Geoff raises an interesting point about high power demand scenarios which may exceed reserve power which in turn cause temporary failure of the camera. I guess one way to see if this is the cause is to swap in the AC-DC power unit and trying out similar image taking scenarios which tax power.
I have looked on the net for other that have experienced image corruption and there was one individual who was trying to taking bracketed panoramas that induced a similar issue. Here is that URL: http://www.dpreview.com/forums/thread/3500177
And Dovey's posting here (scroll down): http://www.fredmiranda.com/forum/topic/1150855/74
Though I found someone else who says they see corrupted image with apparently no battery or card problem: http://www.dpreview.com/forums/thread/3509747
And some one on this thread interestingly observes:
The fact that JPG is fine suggests that the problem is not battery charge nor SD card. (I ran Windows chkdsk on my card and my hard drive; no problem.) The constant pattern of failure - magenta and green divided by a horizontal line - strongly suggests a trouble point in the firmware processing that writes the strictly raw portion of the X3F file.
The questions for Sigma Corp.: what is this error, and is it initiated by a characteristic of the data of a shot or by something in the camera environment, such as the user pressing a certain button at a precise moment while the preceding shot is still being processed?
Were those five shots together in sequence or scattered over the 78?