go version = 1.6
go env = windows, 386

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

The base64 encoding (without padding): of 1 bytes gives 2 chars; of 2 bytes gives 3 chars. And the reverse mapping is true (2 chars decode to 1 byte; 3 chars to 2 bytes).

base64.RawURLEncoding.DecodedLen(2) returns 2, but should return 1.
base64.RawURLEncoding.DecodedLen(3) returns 3, but should return 2.

The bug is the calculation for the NoPadding case in DecodedLen:
WRONG return (n*6 + 7) / 8
RIGHT return n*6 / 8
https://golang.org/src/encoding/base64/base64.go