(This new issue extracts a sub-issue that was originally mentioned in #5128 (comment). After investigating the cause, I've realized this issue is different and smaller in scope than the original issue #5128, that's why I'm making this new issue.)

An instance of gofmt not being idempotent. Repeatedly invoking gofmt will add a level of indent to the b string line each time:

http://play.golang.org/p/Pwn4cOHtMN

I've identified the root cause in go/printer package (therefore the bug affects cmd/gofmt, go/format and go/printer packages). It's due to an unhandled edge case in stripCommonPrefix func.

It happens when a comment block consists of a first line with /*, 1 or more blank lines, last line with */. The internal prefix variable ends up never being set to a correct value before being used (it's not set at all, so it always has zero value, the empty string). See #5128 (comment) for more details.

I already have a fix and test cases done locally, I will submit a CL that closes this issue shortly.