Why is "drush cc all" failing with "undefined function user_load()"
The problem went away after I restarted memcached.

I discovered that by running drush with -d and seeing a PHP error about a $theme value not being a valid index. Tracing the code showed that access to cache_bootstrap was getting empty arrays for the module and theme data.

From now on I plan to restart memcached or flush its cache on my test server after rebuilding its database. (I've done this kind of rebuilding dozens of times before with no issues.)