On Czech/Korean wikis, we've seen ~65 occurrences of this message (out of 570 distinct users). There is only a single message per user ID. The error is caused due to PageViews->setUserHashingSalt() happening in the context of a deferred postsend update, so when the user GETs the WelcomeSurvey the hashing salt hasn't been initialized yet. The practical implications are that the hashed values in the first two events may not be consistent for these users.
The fix is to initialize the hashing salt immediately rather than in a deferred update.