Found this similar issue from a few months back:
http://www.minecraftforge.net/forum/index.php?topic=7846.0
Was not able to get as consistent results, but it is definitely client-side. I've had three separate commandblocks have their text completely vanish from the GUI. Logging out and back onto the server resolved the issue, but it is rather annoying trying to get work done. Did not happen until I installed Forge.
Mods running:
Rei's Minimap
In-Game NBTEditor
The server is vanilla Minecraft 1.6.2. All functionality still works, just can't see the text in the blocks. Does seem to be related to rapidly opening/closing the GUI on CommandBlocks currently receiving/executing their commands. All the affected blocks thusfar have been attached to 3-tick redstone clocks.
By no means is this game-breaking for me, the Forge mods are purely for convenience, but I know it may affect others. Thanks!