The new renderer override and template used for Boost changes the classname of fake blocks from .fake_block to .fake__block (with a second underscore).

It does this as it ignores the class set by the old code and builds the class in the template from

block_{{{type}}}

and "type" in this case is "_fake".

This broke my theme CSS that targetted fake blocks, though I couldn't find any core CSS that relied on this class.

I did find one bit of core JS that seems to expect that class though:

lib/yui/src/blocks/js/manager.js

103:                invalid: '.block-hider-hide, .block-hider-show, .moveto, .block_fake',

It seems like it's intended to stop drag handles being put on fake blocks.