I'm pretty certain this is a bug, rather than me just being stupid:

Trying to edit the relationship for the orchestra performing on the recording http://musicbrainz.org/recording/395ac93e-a5c8-451b-8a10-c5bc39552533 to add a date (1979). The resulting edit is http://musicbrainz.org/edit/14788801 - "This edit failed due to an internal error and may need to be entered again."

I don't think I did anything unusual: just changed the contents of the date field and hit submit. I note that I was able to add a date for the conductor, so maybe this is something weird with the New York Philharmonic artist?