For example, with Claude, I have an "operations" project that naturally grew to cover daily use of shared family calendar, sweeping my mail inbox, and my current personal todo lists, but also a lot of the latter made it deal with my Home Assistant instance. I have separate project for specific things to do with Home Assistance (e.g. one that's about "life support" - HVAC controls, dashboards, monitoring, etc.), one about phone specifically (front-loaded with dumps of specs of my phone's hardware, OS, etc.). Each of them has its distinct set of memories accumulated over months.
And so every couple sessions, I hit a situation in which the agent has to interact with tools and rulebooks that are focus of a different project, and it fumbles a lot. E.g. HA Life Support needs to add some tasks to the todo list, or the Ops project needs to look up climate stats for some reason or other, etc. In these moments, I really wish project memories could mix - but they can't, the boundary is high.
The most annoying case is when I tell Claude that it's wrong, and we literally worked out a solution (or consensus on ethics) in a recent conversation - and then it spends couple minutes looking through past history, burning a chunk of my 5-hour limit, only to come back empty. Yep, that conversation happened in another project. *sigh*