cupid - 8:51 pm on Mar 8, 2012 (gmt 0)
we experimented with changing titles/metadesc/metakeywords for 10 second level pages. they were quite general and really old & not relevant (from 6-7 years ago). we lost 30-40% of SE traffic to these pages, been over a month now with no sign of it coming back. the bounce rate is lower, time on page, page views are higher, but it hardly compensates for such huge loss of traffic.
we also did a similar test to 10 third level pages and the results were similar, although not as bad- some gains, some losses, with overall loss of about 20%
lesson learned, not touching any titles ever again...