All it takes is a sufficiently big pile of custom features interacting. I work on a legal tech product that automates documents. Coincidentally, I'm just wrapping up a rewrite of the "engine" that evaluates how the documents will come out. The rewrite took many months, the code uses graph algorithms and contains a huge amount of both domain knowledge and specific product knowledge.

Claude Code is having the hardest time making sense of it and not breaking everything every step of the way. It always wants to simplify, handwave, "if we just" and "let's just skip if null", it has zero respect for the amount of knowledge and nuance in the product. (Yes, I do have extensive documentation and my prompts are detailed and rarely shorter than 3 paragraphs.)