Plain and simple, there are overlapping shortcuts in the Liquify filter window.
The shortcut for the Smooth Tool is the E key, however once you press the ALT key inside the Liquify window, that shortcut will no longer work, it will instead toggle the Show Mesh checkbox. Restarting the Liquify window or even Photoshop doesn't seem to help.
The other pair of overlapping shortcuts is for the U key, which will toggle the Show Guides checkbox if you haven't made any changes to the Liquify mesh, or it will press the Reconstruct button if you did.
This is in the Windows version of Photoshop CC 2015.1.1, it was also there in CC 2015.0.1
David Mohr • Adobe Employee, Jan 14, 2016Jan 14, 2016
Whoops. I think an apology is in order -- I'm sorry, I didn't fully grasp what you were describing. With the clarification, I totally see what you're talking about. And you called it; that IS a real and valid and serious (and quite new) bug in Photoshop. I'm going to log it right now. The crux appears to be that once pressed ALT never leaves the "pressed" state, thereby invalidating a ton of other shortcuts.
For what it's worth, given the number of commands (and limitations of lan...