1. What version of Go are you using (go version)?
   go version go1.6.2 windows/amd64
2. What operating system and processor architecture are you using (go env)?
   set GOARCH=amd64
   set GOBIN=
   set GOEXE=.exe
   set GOHOSTARCH=amd64
   set GOHOSTOS=windows
   set GOOS=windows
   set GOPATH=c:\dev\go
   set GORACE=
   set GOROOT=C:\Go
   set GOTOOLDIR=C:\Go\pkg\tool\windows_amd64
   set GO15VENDOREXPERIMENT=1
   set CC=gcc
   set GOGCCFLAGS=-m64 -mthreads -fmessage-length=0
   set CXX=g++
   set CGO_ENABLED=1
3. What did you do?
   Test decodeLen(encodedLen(x)) != x for various values of x. Only multiples of 3 produce consistent results.

See https://play.golang.org/p/l7lcBKVxDU

It is probably mistaken, e.g.:
encodedLen(1) => 2, ok since you must have 2 6-bit units to accomodate 8 bytes.
decodedLen(2) => should be 1; since you cannot extract 2 full bytes from 12 bits. In actuality, DecodedLen() returns 2.

Correcting this behavior may be bad for backward compatibility.
I suggest alternately to amend the documentation.