Found a couple of references to this elsewhere on the DUC but not a specific thread for it in the TDM forum so I thought I'd start one.
Had this happen yesterday, seemingly without explanation. The MIDI editor in a session would zoom in, but wouldn't zoom out by any method (zoomer tool, shortcuts or the +- buttons in the timeline). The edit window showed the session end had gone to something like 4 hours, with no corresponding region or automation node to explain this.
Fortunately it happened fairly early in the proceedings, so we imported the data into a fresh session, which worked fine.
It may have something to do with legacy sessions, as the session in question had come from someone else's system who had been working with older versions.
If anyone from digi would be kind enough to look into this I have posted the corrupt session here:
http://download.madhat.co.uk/MIDIbug.zip