Feature flag rollout breaks endpoint

At 15:45 your mobile API's error rate jumps to 12% and a specific endpoint, GET /v2/home-feed, starts returning 500s for a slice of users. There was NO code deploy in the last 6 hours. The error logs show a NullPointerException deep in a new 'personalized ranking' code path that's normally dormant. Your feature-flag dashboard shows that 8 minutes before the incident, someone bumped the personalized_ranking_v2 flag from 5% to 50% rollout. The flag's targeting also references a user.cohort attribute that's null for users who signed up before a certain date. The errors correlate exactly with users in that older cohort who are now in the 50% bucket. How do you respond and prevent this?