In the Readme https://github.com/golang/go/wiki/CodeReviewComments#error-strings it states:

Error Strings
Error strings should not be capitalized (unless beginning with proper nouns or acronyms) or end with punctuation, since they are usually printed following other context. That is, use fmt.Errorf("something bad") not fmt.Errorf("Something bad"), so that log.Printf("Reading %s: %v", filename, err) formats without a spurious capital letter mid-message. This does not apply to logging, which is implicitly line-oriented and not combined inside other messages.

The example is bad and the justification given is weak. For example, a sentence case error message parsed by log.Printf("Reading %s: %v", filename, err) might produce the following: Reading file.txt: Something bad. This is perfectly fine English. Programming style conventionally follows American usage, "A colon is not normally followed by a capital letter in British usage, though American usage often prefers to use a capital." Thus, there is no problem presented in the example given, and in fact, American usage should by rights protest the lack of the capital letter. Surely there is a better example that should be given for this rule!

Furthermore, I think it much more likely that the reason for requiring error messages to be lowercase is to avoid the issues that arise if mixed case error messages are acceptable. For example, if one error message is Something bad and another is something bad, an issue occurs that the error messages are different but likely had the same intended meaning. By requiring the case to be one way and not the other, we reduce the risk of accidentally creating new messages.

In conclusion, the rule is good. We should continue to start error messages with lowercase letters, but we should drop these bad examples and (in my opinion) post hoc excuses to justify the rule.