A console error is thrown when trying to add a new block to the editor or when opening an existing post with certain blocks such as gallery and cover image
Situation 1
Situation 2
OS: MacOS High Sierra 10.13.3
Browser: Chrome Versie 63.0.3239.132
WordPress: 4.2.9
Theme: Twentyseventeen 1.4
Gutenberg plugin: 2.1.0
Screenshot for situation 1

Screenshot for situation 2

I'm seeing this too on my latest pull from master. Started a new post and I get this error every time I click on the inserter:

It's weird because it seems to be referencing the button from the demo post...
@youknowriad this seems to be related to the deprecation API for blocks. We need to make sure that we display errors using console.error only when it was not possible to properly parse the block. Otherwise, we should rather use console.info to inform that we updated HTML code to the latest version.
It's weird because it seems to be referencing the button from the demo post...
I think it's referencing to saved reusable blocks.
At least for me it threw gallery error and my only saved block was a gallery. When I deleted it the error stopped appearing.
Closing because I tested with WordPress 4.9.6 and Gutenberg 3.1.1 using Chrome 67.0.3396.87 on macOS 10.13.5 and was unable to see the "Block validation" console error after clicking 'Add block' on a new post or opening the Gutenberg demo post. Please re-open if this problem is still happening for you!
Most helpful comment
@youknowriad this seems to be related to the deprecation API for blocks. We need to make sure that we display errors using
console.erroronly when it was not possible to properly parse the block. Otherwise, we should rather useconsole.infoto inform that we updated HTML code to the latest version.