Cursor_sharing=similar.....failed

Looking for ideas/comments/suggestions..........

We changed cursor_sharing from EXACT to SIMILAR last Saturday morning at 6am.
Did that via an "ALTER SESSION SET....." command. The environment is large (7TB) but not very busy on Saturday morning. We had cursor_sharing=similar in multiple test environments, for many months without issue.

At 9:15am, we started to generated ora-04031 errors. By 9:30am, we could no longer sign-on.
We ended up shutting down the middle-tier, and deleting all of the OS processes on the DB server, that represented connections to the instance.
At that point, we were able to signon, backout the change, and startup the middle-tier again.

In the alert log, we saw the ora-04031 errors, and also many (thousands) of messages saying
"PMON failed to acquire latch, see PMON dump".

The PMON trace file show the same entry, repeated thousands of times:
"PMON unable to acquire latch 2c4cb108 library cache
  possible holder pid = 1000 ospid=42586708"

Any ideas on what happened would be appreciated. Thanks.

Dan Hubler
Database Administrator
Aurora Healthcare
daniel.hubler_at_aurora.org

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