Cactus doesn't break anymore when it grows and there's a glass panel or sign next to it.
This is a bug I found in cactusfarm I made before the snapshot.

I tested it in creative where they do still break when they grow next to panels or signs.
I haven't tried it yet but I assume it's the same for solid blocks and placable items.

They do break off when you destory the sign or panel or replace it (block updates).