Forum OpenACS Improvement Proposals (TIPs): Re: TIP #57 (Proposed) Add caching db_* API (based on Greenpeace/Berkeley work) to 5.2
Another is that with the forums example, adding a post adds an id rather than changing any existing ids so you have to do an explicit flush on the parent_id anyway.
A third really hairy issue is that for queries which involve permissions checks, it would be prohibitive to try to do any real dependency checking; and I given the varying need for security such queries probably should not be cached (or at least there should be some option to disable caching of queries involving such checks). Certainly, the experiences with PermissionsCacheP point to a lot of problems with caching such things.
I also think its really easy to go overboard on dependency checking, I have worked with a couple of different systems that had dependency checking for caching or for lazy evaluation and both had numerous places where checking the dependencies was slower than simply recalculating.
In any case, I definitely approve the tip, although I think it will be a while before we are really happy with how things work.