Certain redstone cirtuits cause severe lag occasionally, to the 0 fps. For example simple redstone clock.

I've tested it in various versions of the game. The issue appears only in prerelease 7 and further up to release candidate 1 and including it. Prerelease 6 does not have such issue: the circuit causes fps to drop occasionally, to 25-30 from 60+, but not to 0-1. I'm attaching videos displaying freezes occuring in prerelease 7, in release candidate 1, and a video displaying absence of the bug in prerelease 6. Also this bug presists in both multi and singleplayer, also I tested it at least on 2 more different PCs and it persists there as well. 

If needed I can provide additional info or run additional tests.

I have uploaded part of world where you can reproduce the issue, however I had to split archive to 4 parts because of file limit. You will need to download all 4 into same folder and extract it with 7-zip or winrar (or any archive extraction program that supports split rar archives). Just press the button on the floor few times or replace it with redstone torch and you will get some freezes. If you spawn somewhere else, teleport to -315 67 23