Invalidated package, what's going on?

Ever wondered why after a package is recompiled that (active?) application sessions error out first time, and then works okay??

It is the ORA 04068. Read the full article on packages here: Source

Package State and ORA-04068

When a package has at least one constant or variable declared at the package level, that package is said to have state or to be stateful. When a session uses a stateful package, Oracle Database uses session-specific PGA (Process Global Area) memory to store that session’s values for all package-level data.
If a stateful package is recompiled, all sessions that have used the package will fail with the ORA-04068 error the next time they try to access the package. That’s because the state of the package in each session is now out of date, so the package must be reinitialized.

Furthermore, once the ORA-04068 error has been raised, the state of all packages in the session—including, for example, DBMS_OUTPUT—is uninitialized. This usually means that the user must disconnect from the session and reconnect in order to use the application successfully.
The potential for ORA-04068 errors has meant that when IT departments must upgrade the code in their application, they need to first have all users log out from the application. In the 24/7 world of the internet, however, this is not something many companies can afford to do. (etc)...