Okay, so I'm in the middle of a session, recording some vocals in comping mode.  All is well.  Then Sonar X3 Producer (Windows 8.1) starts getting very weird in recording.  Instead of creating new take lanes, it just starts overwriting all the preexisting stuff.  
 
Needless to say, I'm disgusted.  Is there a fix?  Are there perhaps any settings I've messed up?
 
Thanks to all.

Right click on the record button and make sure you are still in comping mode. Cheers.

Yeah, that was my first move.  I should have said that.  No joy.  Closed Sonar, restarted, still the same problem.  Could it be that I've run out of lanes?  Is that even a limitation in SX3P?

I create TONS of lanes when I'm working/testing out song ideas and I've never had that happen. Perhaps the project has been corrupted. Have you been using Save As to create versions while you work? Perhaps you can go back to an earlier version.
 
Another thing I'd look at is all the possible Arm statuses. Maybe you have the record button armed right on one of the lanes which I believe would overwrite despite the current mode (but I never use that button). I'm sure you would notice that but you never know... sometimes simple things get overlooked when we're deep in the thick of things.
 
If it persists I'd contact Cake. Maybe you found a bug. Good luck.

That really does sound you are in record "overwrite mode" and probably hit a short cut key by accident to change it from where it was before.
 
If not, try a new session and record to see if the behaviour is the same on a new session.