Let me share a debugging nightmare with you. We had a multithreaded system written in C++. The state was carefully shared, with concurrent access protected by explicit mutex locks. A team member--ugh--forgot to acquire a lock on a shared data structure and all hell broke loose.
The team member was a senior programmer; he knew what he was doing. He just forgot the locking. It took us some nights full of stack trace to figure out what the issue was.
Writing concurrent programs using shared memory communication can very easily go wrong.
In the book Java Concurrency in Practice, the authors show us how easy it is for internal mutable state to escape (http://jcip.net/ ). Tools, such as Eclipse, make it easy to generate getters, ...