I told a client we'd handle their lead gen then had to eat my words

Back in March I had a prospect who kept asking about guaranteed appointments and I kept saying our B2B outreach was solid. We signed them for a 3 month trial at $2k per month based on my sales pitch. First 2 weeks the email open rates were garbage and I started sweating because I had already spent their deposit on a new CRM tool. Turns out the problem was I was using a crappy list provider that had stale contacts. Swapped to a verified list from a local data guy in Austin and our reply rate went from 0.4% to 3.1% by week 6. Client renewed for another quarter after that and now I always test a small batch before promising anything. Has anyone else had a similar wake up call with list quality vs messaging?