A new bug in version 14w29b:
When you mine a block, it sometimes does not disappear instantly, but after 1-20 seconds. The item drop is there though, and the mined block does not have a hitbox, so it's only a visual bug.
When mining a line of 10 blocks,
When you place a block, it also does not appear instantly. When you hover the "invisble" block, you can see the transparent border that symbolizes that there is a block.

Video showing the bug (with actived debug screen): https://www.youtube.com/watch?v=jz77EXGnHRQ

It occurs with and without "Use VBOs" enabled.