A route map change leaks ISP1's routes to ISP2 and the session drops

R1 (AS 64500) takes default plus partial routes from ISP1 (AS 64496), about 1,870 prefixes, and the same from ISP2 (AS 64497). At 10:05 an engineer replaced the outbound route map toward ISP2 with a new one, ISP2-OUT, to add AS path prepending. At 10:07 the ISP2 session went down and has stayed down. ISP2's NOC phones: "your session sent us more than ten prefixes, our limit shut it". You are now running with no backup ISP.