I've lost a many months session with +290 tabs recently and it was so frustrating to know that there is no way to restore them. I can't use history to get them back because the history was cleared recently and I've been using delayed tab loading, so only a few of them was opening between sessions.
My laptop ran out of free disk space and my Opera had freeze for some time and then I decided to force close its process to open it again without knowing I had not disk space. It was my fault to not make a regular backup of my profile, it was my fault to not use Opera Sync, it was my fault to close the process without knowing I had no free disk space left, but I guess it was not my fault the fact that the "Last Session" file have got truncated.
It would be way better to have the old session file than losting everything with a truncated file. It would be VERY VERY VERY nice if Opera had a way to auto save the session state in a second file and write it back to the main session file when needed and, in both cases, save them atomicaly instead of truncating and overwriting - so that you save the contents to a temporary file and later replace the main file with it, so if anything goes wrong the file get untouched. If the main "Last Session" file get lost in some way, you can still recover the "Auto Saved Last Session" file. Please, someone file this request to the development team. This would save lives. 