For me it's when I get assigned to a project where we're rewriting a perfectly-good app for any of the following reasons: (1) in order to use the latest buzzword (kafka... fsck me) (2) as a PR stunt to get the company name back in the press (new! and improved!) (3) because some micromanaging activist investor said so (4) as a hail-mary attempt to erase technical debt (5) as a sneaky attempt to erase institutional knowledge so that they can offshore the whole operation (6) so the tech lead has another badge on his resume