When you break multiple instant breaking blocks, breaking effects duplicate (both sounds and particles). This is really annoying as I use the sounds a lot to orientate myself while building. This is a video I made in an earlier snapshot, but it's replicable in 1.4.2: https://www.youtube.com/watch?v=fDMfxjkIRss
Environment
Windows 7 Home Premium 64-bit, Java 7 Update 7 (build 1.7.0_07-b11)
Confirmed, the video accurately depicts the bug in question on 1.4.2-pre.