Context pollution and rot are probably more important than memory, because facts can usually be retrieved if the agent is good at following breadcrumbs.

What's also the biggest killer is code rot. Agents are particularly good at death by thousand cuts. They implement something poorly, or incorrectly, or introduce a bad pattern into the project. Then they continue to amplify that badness over time, as they continue to copy from it on subsequent work. It spreads like a virus.

Keeping these seeds out of the project is very difficult, and cleaning up the rot is very difficult. It also seems like a hard problem to solve because following the existing codebase is something that is good when the code is good, but bad when it is bad. So, seemingly, the solution means more thinking and evaluation for every change that is being made.