I hope nobody minds posting in an old thread, as there's some useful discussion here that would be missing if I created my own new one. I'm experiencing this issue with ShowCaseStandalone and ItemCase which both server-side plugins which feature floating "display" items that cannot be picked up. With a vanilla Minecraft client they appear fine, but with a Forge client these items are invisible. Note that I am using the latest version of Forge to connect to a Spigot 1.8.8 and I am not using any other mods.
In a ticket submitted to ShowCaseStandalone the developer stated that this was a Forge issue and had the following to say:
"first of all, this is a forge related issue or at least an issue on the client.
For various purposes the item count inside of an ItemStack (that is displayed to you)
is set to 0, so that even if there are glitches that allow a player or a mob to pick it up,
they can't do anything with it, since there is no actual item that can be used. I think
forge is, while trying to optimize the frames count, not displaying stacks with 0 items."
http://dev.bukkit.org/bukkit-plugins/scs/tickets/830-not-compatible-with-forge-glsl-shaders/
Given that ShowCaseStandalone states that this is a Forge issue, and Forge states that it must be caused by other mods, what resolution can be reached for this issue?