What I expected to happen was:

The piston or sticky piston to extend the block in its normal state.

What actually happened was:

See here: http://gfycat.com/SameRadiantBlackbird

The piston or sticky piston will throw blocks of meta data 6,7,and 14 into an invalid block state ultimately causing the following and recurring log errors attached on smp.

It appears for a split second the meta data changes back to 0 from 6,14, or 7. Blocks will exhibit behavior as the following gif I recorded. It's quite clear that red and pink wool will change to white wool for a second.

Complex machines such as wither skeleton farms, crusher systems, enderfarms, and witchfarms can cause servers to crash or clients if blocks of meta data values 6,7,14 are used.

How you reproduce this:

Go onto to a server whilst viewing the console or enter a minecraft world and push any block from the list below with a piston:

The following list may be missing a few blocks but there's a pattern with meta data values

Red Carpet (14)
Pink Carpet (6)
Red Stained Glass Panes (14)
Pink Stained Glass Panes (6)
Red Stained Glass (14)
Pink Stained Glass (6)
Grey Stained Glass (7)
Polished Andesite (6)
Red Stained Clay (14)
Pink Stained Clay (6)
Red wool (14)
Pink wool (6)
Nether brick half slab (6)
Gray stained clay (7)
Gray wool (7)
Quartz half slab (7)
Birch wood (6)