[lectrick]

I have seen showstopper, up-all-night bugs with BOTH shared mutable state (see: the reason it is nearly impossible to reduce a certain Rails app's test suite runtime below about a half hour without significant rewriting) AND single threaded mutability bugs (in one case resulting in me taking a month to fix a login bug that nobody else at the company was able to find out in over a year... It was a hash mutation deep inside a middleware affecting auth cookies)

A simple example of a single threaded mutability bug is passing an object by reference to a method which then (either intentionally or unintentionally) mutates the object, to the complete ignorance of the caller up in the call stack. The only thing stopping that is literally, programmer skill. No language constructs. At least in all the OO langs I ever worked with.

[ubernostrum]

You really should try out Rust. The scenario you describe would not compile in Rust.

[lectrick]

I have been meaning to. I hope great things come of it.