http://play.golang.org/p/foh_Gc09Ls

package main

import (
        "bytes"
        "fmt"
)

func main() {
        var buf bytes.Buffer
        buf.WriteString("abcdefghijklmnopqrstuvwxyz")

        line, err := buf.ReadBytes('m')
    fmt.Printf("%q %v\n", line, err)

        err = buf.UnreadByte()
        if err != nil {
                panic(err)
        }
}

"abcdefghijklm" <nil>
panic: bytes.Buffer: UnreadByte: previous operation was not a read

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