From doing Google searches, I can see this had also been a problem a year ago, but I haven't noticed it until just with this "snapshot" version. It must have been a bug that was fixed, but recently was inadvertently unfixed when the new features were added in this snapshot. It causes some cacti to be completely invisible except for the top block. The invisible blocks usually can't be hit and destroyed, but if you touch them you take damage. You can destroy the whole cactus by digging out the block of sand below the cactus. This is the only way to destroy the entire cactus (including all invisible segments) if it is suffering from the invisibility bug, as destroying the top segment (the only visible part) doesn't destroy the invisible part (the rest of the cactus).

Bug Report Update:
If the cactus has loaded correctly in a previous version of Minecraft, it will load correctly when upgrading to the new snapshot version. However if you are rendering the level for the first time using the new snapshot version this bug effects a large number of cacti. To fix it, switch back to an older version of Minecraft.jar (I kept a backup of 1.4.7 before upgrading to 13w01b) and load the level so it loads the cacti properly, and then switch back again to 13w01b, the cacti that rendered properly in 1.4.7 (and thus saved to the level file as correctly rendered) will be loaded properly in 13w01b when your level is loaded back in in this snapshot version. As a result it is possible to "fix" the cacti by temporarily reverting to an old version and running through the desert to make sure the cacti are rendered properly as the level is generated, and then re-upgrading to the new version.

I have attached a zip file with some screenshots and info in a txt file with useful info (including level seed, and location information for the screenshots, which themselves have lots of useful info as I displayed the F3 debug overlay when doing the screenshots).