Only one engineer understands the legacy billing schema

If the one engineer who knows the legacy billing schema is unavailable during the migration window, then nobody can resolve mapping errors and the data cut-over stalls.

Impact if it happens — migration blocked for as long as it takes to onboard a replacement, realistically three weeks.

Response (mitigate) — pair a second engineer on every billing ticket from sprint 9, and get the schema mapping written down rather than explained.

Trigger to watch — the second engineer still cannot resolve a mapping bug unaided by the end of sprint 11.