In my humble opinion, a lot of projects go quietly bad when they experience some sort of new requirement that gets underestimated in terms of its architectural impact by project management. At such time senior Devs have either already moved on, or have their eye off the ball such that new features get incorporated without the necessary architectural support. These inflection points can themselves introduce complexity but often become the gateway for all sorts of subsequent small things that explode in size. In short, don't miss architecture moments