The input format of the xargs command doesn't match what any other command produces. Yes, it's bizarre. With -I, xargs ignores indentation, which is why the file names with initial spaces are mangled. Do not use xargs except with the -0 option or when you know your input doesn't contain characters that would confuse it.

I agree with this statement so much. We should absolutely be failing hard rather than forcing people to debug thread safety issues at runtime. I can't think of anything more infuriating than debugging an issue that happens "sometimes".

The problem is that in the case where an app is multi-threaded, and we don't switch off autoload, the case would be that it probably won't blow up, but random stuff will mysteriously sometimes fail in weird ways. So ask yourself this, what would you rather want, option 1) where you can get an exception at runtime, or option 2) where you get random, unpredictable, weird, hard to explain, difficult to debug bugs at runtime. Personally, I'm going to choose option 1. The downside of thread-safety issues is so much worse than the downside of the possibility of an exception. The way you're handling it makes it sound as though thread-safety is not important, as though Rails is still optimizing for the single-threaded case. That seems like a huge step back.

paranoia has some surprising behaviour (like overriding ActiveRecord's delete and destroy)

I've worked with and have helped maintain paranoia for a while. I'm convinced it does the wrong thing for most cases. Paranoia and acts_as_paranoid both attempt to emulate deletes by setting a column and adding a default scope on the model. This requires some ActiveRecord hackery, and leads to some surprising and awkward behaviour.

You can imagine how big of a surprise was finding out that model/character being a child of model/node causes an error when we import model/position.

Basically, the idea is that a train tried to start with the caboose brakes stuck on. After releasing the caboose, the train still could not start. The problem was that when the train attempted to start with the caboose brake on, it stretched all the inter-car couplings so that the whole train was just like one big car. At this point, the friction from the engine train wheels was not enough to get the whole thing going. Instead, you need to just get one car moving at a time - this is why there is space between the couplings.

Capybara and Webrat use some of the same method names which leads to runtime collisions that reveal themselves in confusing ways.