Sitecore V5 implements cache for reading. When several Sitecore installations point to the same databases (Dirty databases in our case), the developers may see invalid content. This is caused by the fact that caches (as well as sessions) are handled separately for each installation.
Every code recompilation causes site restart, which clears the cache. Code is recompiled quite often during the development process, that’s why the problem described above may not show up very often. But it may be really severe when several developers install packages simultaneously or create items programmatically.
That’s why the Clean database should only be modified with the Development server Sitecore installation. For instance, when several packages are installed by different users simultaneously within one Sitecore installation, package installations are queued and no database breach occurs.