What version of Go are you using (go version)?

$ go version
go version devel +4cce27a3fa Sat Jan 21 03:20:55 2017 +0000 linux/amd64

What operating system and processor architecture are you using (go env)?

$ go env  # abridged
GOARCH="amd64"
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"

What did you do?

https://play.golang.org/p/gyq-AzXEXg

fmt.Printf("%.e\t", 1.)
fmt.Printf("%#.e\n", 1.)

fmt.Printf("%.f\t", 1.)
fmt.Printf("%#.f\n", 1.)

fmt.Printf("%.4g\t", 1.230)
fmt.Printf("%#.4g\n", 1.230)
fmt.Printf("%.4g\t", 12.30)
fmt.Printf("%#.4g\n", 12.30)
fmt.Printf("%.4g\t", 123.0)
fmt.Printf("%#.4g\n", 123.0)

What did you expect to see?

1e+00	1.e+00
1	1.
1.23	1.230
12.3	12.30
123	123.0

This is what the analogous C or C++ program produces.

In C or C++, the # flag for the floating-point (a, A, e, E, f, F, g, and G) conversions specifies that

the result of converting a floating-point number always contains a decimal-point character, even if no digits follow it. (Normally, a decimal-point character appears in the result of these conversions only if a digit follows it.)

In addition, for the g and G conversions, it specifies that

trailing zeros are not removed from the result as they would otherwise be.

Since the formatting of Go's fmt package is analogous to C's stdio functions, it's naturally expected that the former would respect this aspect as well. In particular, in a scientific context, it's crucially important to have "%#.4g" produce the same number of significant digits, either for 123.0 or 123.4; formatting 123.0 as "123" or "123." would imply only 3 significant digits, instead of 4.

What did you see instead?

1e+00	1e+00
1	1
1.23	1.23
12.3	12.3
123	123

The # flag doesn't have any effect, and the decimal-point characters and the trailing zeros are removed the formatting result.