This seems to be consistent with the documentation (https://godoc.org/fmt):

• int: # for 0x for hex (%#x) and 0 for pad with leading zeros rather than spaces (padding after the sign).
• string: %x base 16, lower-case, two characters per byte then # for 0x for hex (%#x) for each byte and pad the string with leading zeros rather than spaces.

The difference is that the verb %x applies to each byte in strings and # is an alternate format for the verb so also per byte but the padding is not described to be per byte but applies to the output as a whole.

An improvement for the documentation that could be made is for '0':
instead of 'for numbers, this moves the padding after the sign' it could say 'for numbers, this moves the padding after the sign and leading 0x'

But it does not add an '0x' for each byte:
Example on Playground

> 00000x3031

As far as I understood you it should print something like:

> 000x300x31

you are right it i was confusing this with ' ' where it applies per byte and is mentioned explicitly "% x" in documentation.

Referring back to the original issue of 0 padding I think its still in line with documentation that padding of 0 applies to the whole string after it has been formatted? Could maybe made explicit.

To format the string like the number in the first example you could do:
fmt.Printf("0x%08x\n", "0");

If its not an obvious bug or contradicting documentation its likely not to change how fmt formats this as its established behavior by now that other programs will rely on.

Referring back to the original issue of 0 padding I think its still in line with documentation that padding of 0 > applies to the whole string after it has been formatted? Could maybe made explicit.

I cannot find this in the documentation for printf.

This is what I've found:

 ' '	(space) leave a space for elided sign in numbers (% d);
	put spaces between bytes printing strings or slices in hex (% x, % X)
0	pad with leading zeros rather than spaces;
	for numbers, this moves the padding after the sign

It does only say what happens if you pad it with spaces, so I agree it should be more explicitly stated what happens if you use zeros instead of spaces.

CC @robpike

Padding applies to the whole result.