It seams to concern only my installation. I use Windows 7 with lighttpd, sqlite and php-fpm from Cygwin.
When I enable debug mode I get the following message 8 times on the described situation.
[objectcache] Rejected set() for global:user:id:wiki:1 due to pending writes.
I suspect that you'll have to do the legwork to figure out what's going on here, unless someone else comes along who can reproduce it.
That key looks like it's coming from the User class caching stuff, not Session stuff.