I tend to be a little obsessive about the accurate reporting of place and time (as a corollary to my obsession about the accurate reporting of fact). For that reason, twice a year (at the beginning and end of DST), I photograph a display of my presumably-accurate computer clock, before and after I have reset my digital camera's date/time setting to the correct time, so that if ever I might wish to report the actual time that a photograph was taken, I can translate the camera-clock time (in the EXIF data) into real time.
For example, when I did this album of a Toronto Wildlife Centre goose rescue, I had to subtract 2 minutes and 34 seconds from the photos' time stamps because the camera clock had gained that much since I last calibrated it in March.
By coincidence, it was back in March when my iPhoto additions first no longer appeared in the 'photos' section of the application. The pictures were still accessible through the 'events' section and I put the matter down to an application bug resulting from the sheer number of photos (over 44000) it had to display on my aging infrastructure.
This morning, I noticed that the date stamp for a recent picture had the year as 2011. In the four-and-a-half months since I last calibrated the camera clock I had never noticed that the year was incorrect. The photos had in fact all been added to the 'photos' section of iPhoto, but they were back in 2011 where I never once occasioned to look. Fortunately it was easy to correct all of the bad dates in a batch date/time adjustment.