Adventure mode animation not playing after 1 swing when breaking insta-breakable blocks

I created a LAN World and joined on my other computer as Adventure mode in the 1.5 snapshot on Mac OSX. I broke grass and the animation stopped after one swing. I walked around with the left mouse button still clicked in breaking grass with no animation playing other than my character walking. I was able to recreate this issue in 1.4.6 on Windows 7.