Minecraft 14w11b, Minecraft 14w25b, Minecraft 14w26c, Minecraft 14w27b, Minecraft 14w32a, Minecraft 14w32b, Minecraft 14w33a, Minecraft 14w34d, Minecraft 1.8-pre1, Minecraft 1.8-pre2, Minecraft 1.8, Minecraft 1.8.1, Minecraft 1.8.8, Minecraft 15w31b, Minecraft 15w50a
Java 1.7.0_55 on Windows 8.1 x64
Java 1.8.0_11 B12 on Windows 8.1 x64
Pistons extending upwards break sand and gravel the same way a torch breaks falling sand and gravel.
I have included a world download with a machine to demo the behaviour. The bug is most distinct with the central repeaters on the second setting, but the first setting will result in the block breaking eventually. To start the machine, load sand or gravel on the pistons and press the stone button.
Please note, this may be (or may not be) related to bug
MC-46225, but this is different behaviour. In snapshots up until 14w10c the piston disappears, but from 14w11b the piston no longer disappears, but now the block breaks as described above.
Further info - if you replace the pistons with sticky pistons in the test world attached, the problem will not manifest until you change the repeaters to the first setting. On that setting the behaviour of sticky and non-sticky pistons is identical.