This seems to me to be the same class of error as those that aren't so much a go thing as a very comma gotcha for closures; you're placing x in the scope of the for loop and passing it into a series of anonymous functions, so it gets closed over and referenced as a common variable between each of those functions; thus before each goroutine has a chance to run the loop has completed and x == 3. You'll find this behaviour in any language with closures.

In the second example you're allocating a new local variable on each iteration, so each individual value gets closed over separately. That's probably not what you'd want usually, hence that not being default behaviour.