Are you sure this isn't just the result of mixing stderr and stdout differently?

Try it using only one print function.

Yes, indeed. I'm closing this now. Please mark as “invalid.”

It's still weird that I cannot reproduce this behaviour without deference. Go seems to obey the order in which I output things in other cases.

Reopening with new title.

I think the playground's hooks for intercepting os.Stdout and os.Stderr don't work for the builtin println function, because that writes to stderr directly, without going through the playground's hooks.

These work, using only one style of printing:

http://play.golang.org/p/c0XFpaPnh8 -- with println (stderr)
http://play.golang.org/p/M0y2nixfak -- with fmt.Println (stdout)

This is wrong, or at least confusing:

http://play.golang.org/p/L0ys_wO462

/cc @adg (who's on vacation for ~5 more weeks)

package main

import "fmt"

func main() {
	defer println("D")
	defer fmt.Println("C")
	println("A")
	fmt.Println("B")
}

This runs correctly on playground: http://play.golang.org/p/L0ys_wO462

A
B
C
D

Same is valid for the program on the top.

/CC @bradfitz

After running couple of more attempts to break on playground, I can say that this works as designed. We can close it.

Closing as
http://play.golang.org/p/c0XFpaPnh8 -- with println (stderr)
http://play.golang.org/p/M0y2nixfak -- with fmt.Println (stdout)

and the mixed version http://play.golang.org/p/L0ys_wO462

all have the same output of

A
B
C
D

Thanks @andybons !