at least go1.4, go1.6, go tip:

https://play.golang.org/p/tArTXnDSma

package main

import "fmt"

func main() {
    fmt.Printf("%06v\n",[]interface{}{-10.0,10})
    fmt.Printf("%06v\n",[]interface{}{-10,10})
}

Output:

[-00010 00010]
[-00010 000010]

They should be the same.
However formatFloat alters the width field and does not restore its original value.

Discovered this while writing test cases for a float format cleanup.
Which failed for the current go but worked for my new code.

Will submit a separate fix for this referencing this issue first.