Session confusion using mem_cache_store?

I have a Rails 2.3 application with about 45,000 active users. We currently store sessions in the database with active_record_store.

I recently tested switching the session store to memcache, but within a day of it running, we had a few users write in saying that they logged in and saw other users' data. I switched back to active record, no more complaints. That reminded me that when I had tried this a year or so ago, I'd seen a similar problem.

Does anyone have any idea why that might be happening? Here's the section I was using in config/environments/production.rb

require 'memcache' memcache_options = { :compression => true, :debug => false, :namespace => :myapp, :readonly => false, :urlencode => false }

CACHE = memcache_options CACHE.servers = 'localhost:11211'

ActionController::Base.session_options[:cache] = CACHE

config.action_controller.session_store = :mem_cache_store