What does 'go version' print?
go version go1.3beta2 +708e129e91a9 Wed May 21 16:01:54 2014 -0700 linux/amd64

What steps reproduce the problem?
Run this program on a 64 bit machine.

package main

import (
        "bytes"
        "encoding/gob"
)

func main() {
        // Encode an int slice.
        b := new(bytes.Buffer)
        t := []int{1, 1, 1, 1, 1, 1, 1, 1, 1, 1}
        gob.NewEncoder(b).Encode(t)

        // Set an invalid value to the first byte of the length field for the slice.
        // This makes the encoded slice very long.
        buf := b.Bytes()
        buf[len(buf)-len(t)-1] = 0xfa

        // Decode panics on 64 bit.
        u := make([]int, 0, 1)
        gob.NewDecoder(b).Decode(&u)
}

What happened?
It fails with the following error.

panic: runtime error: runtime: allocation size out of range [recovered]
        panic: runtime error: runtime: allocation size out of range

goroutine 16 [running]:
runtime.panic(0x4e3880, 0x5a2da7)
        /usr/local/google/home/ruiu/go2/src/pkg/runtime/panic.c:279 +0xf5
encoding/gob.catchError(0xc2080241c8)
        /usr/local/google/home/ruiu/go2/src/pkg/encoding/gob/error.go:38 +0x8f
runtime.panic(0x4e3880, 0x5a2da7)
        /usr/local/google/home/ruiu/go2/src/pkg/runtime/panic.c:248 +0x18d
reflect.unsafe_NewArray(0x4b66c0, 0x20202020202, 0x0)
        /usr/local/google/home/ruiu/go2/src/pkg/runtime/iface.goc:596 +0x3a
reflect.MakeSlice(0x7fc55c9c5208, 0x4af9e0, 0x20202020202, 0x20202020202, 0x5a5f80,
0xc2080224b0, 0x7fc55c826be8, 0x7fc55c826be0)
        /usr/local/google/home/ruiu/go2/src/pkg/reflect/value.go:2227 +0x1e7
encoding/gob.(*Decoder).decodeSlice(0xc2080240f0, 0x7fc55c9c5208, 0x4af9e0,
0xc20803a400, 0xc208042d20, 0x526e58, 0x8, 0x1, 0x0, 0x7fc55c9c11b0, ...)
        /usr/local/google/home/ruiu/go2/src/pkg/encoding/gob/decode.go:672 +0xc2
encoding/gob.func·003(0xc208023380, 0xc20803a400, 0xc20803c2d0)
        /usr/local/google/home/ruiu/go2/src/pkg/encoding/gob/decode.go:892 +0xe0
encoding/gob.(*Decoder).decodeSingle(0xc2080240f0, 0xc208042aa0, 0xc20803b1c0,
0xc20803c2d0)
        /usr/local/google/home/ruiu/go2/src/pkg/encoding/gob/decode.go:487 +0x1b7
encoding/gob.(*Decoder).decodeValue(0xc2080240f0, 0xc200000041, 0x4aadc0, 0xc208042d20,
0x0, 0x160)
        /usr/local/google/home/ruiu/go2/src/pkg/encoding/gob/decode.go:1250 +0x441
encoding/gob.(*Decoder).DecodeValue(0xc2080240f0, 0x4aadc0, 0xc208042d20, 0x0, 0x160,
0x0, 0x0)
        /usr/local/google/home/ruiu/go2/src/pkg/encoding/gob/decoder.go:229 +0x195
encoding/gob.(*Decoder).Decode(0xc2080240f0, 0x4aadc0, 0xc208042d20, 0x7fc55c9c5418, 0x0)
        /usr/local/google/home/ruiu/go2/src/pkg/encoding/gob/decoder.go:204 +0x1fb

What should have happened instead?
It should not panic.