Users of memcache need to guard against hung applications. The keys used to identify stored data in memcache must be 250 characters or less and must not contain whitespace characters or control characters.
Programmers using memcache-client, Eric Hodel's popular memcache client library for Ruby, should be especially careful as memcache-client does no escaping to protect against this.
Thanks to Zed Shaw for getting the word out. Thanks also to Eric for a quick confirmation of the potential problems.