We are using CiviCRM 4.6.9, and Drupal (where do I find version #??)
The screenshot is from my dashboard; the message also appears when attempting to view or edit most contact records. Most but not all, and I haven't found any pattern.
As far as I know, we have not recently updated Drupal or Civi.
Why are we suddenly getting this message? What can I do?
More information: the problem has gone away all by itself, so I still don't know what really happened. And that's OK by me for now.
Thanks for the helpful answers - I found the file with backtrace, and also found where to turn debugging & backtrace on and off for next time. So a helpful learning experience.