Since there are so many pictures, and I want them in a specific order, I have uploaded them to a github repository and will link them below.
In Minecraft Windows 10 Edition, version 1.2 beta number 3, creative mode, numerous blocks are missing from the inventory. With these same blocks, if you enter them in a /give command, it comes up with a syntax error. Interestingly enough, it is still possible to get these blocks. How, you ask? You find the block that was already placed. For example, if this is an old world from a previous version and you already placed dispensers, you can find that dispenser, middle click it, and it will be added to your inventory. If it is not already in the world, you're helpless, sorry!
Murtag has already stated that he is unable to reproduce this, but the screenshots should act as proof that this is an unfortunate error. I will soon restart my minecraft (at the moment others are playing on the world) and will edit the post afterwards.

Example 1: Dispensers
Dispensers cannot be found in the inventory
(pic: https://github.com/TheRoyalBlock/Minecraft-Bug-Report-Photos/raw/master/3h.jpg)
I then attempt to get a dispenser by using the /give command
(pic: https://github.com/TheRoyalBlock/Minecraft-Bug-Report-Photos/raw/master/4h.jpg)
And I end up getting a syntax error
(pic: https://github.com/TheRoyalBlock/Minecraft-Bug-Report-Photos/raw/master/5h.jpg)

Example 2: Rails
As I attempt to find regular, powered, activator, or detector rails, I look through the inventory.
I come across rails.
(pic: https://github.com/TheRoyalBlock/Minecraft-Bug-Report-Photos/raw/master/4i.png)
I then expand the rails, but, to my disappointment, I find only regular rails.
(pic: https://github.com/TheRoyalBlock/Minecraft-Bug-Report-Photos/raw/master/5i.png)
I search for rails, and again, find regular rails again. SIDE NOTE: This pic has the build info as well, as requested by Murtag.
(Pic: https://github.com/TheRoyalBlock/Minecraft-Bug-Report-Photos/raw/master/7i.jpg)

Example 3: Anvils!
This strays from the original topic a little bit, but is still a small inventory error.
As I decide to look for the different types of anvils in the inventory (adds a visual aspect), I find anvils, 3 of them! One of them is an expandable menu, and there are two more. All three of them have the same exact "anvil" tag, and the exact same item image. They are also all slightly transparent. How strange!
(pic: https://github.com/TheRoyalBlock/Minecraft-Bug-Report-Photos/raw/master/4j.jpg)
Here's another pic, this time with the stuff circled. You see the arrow inwards, showing that the menu is not expanded. You also see the two other anvils, with the anvil tag and transparency.
(pic: https://github.com/TheRoyalBlock/Minecraft-Bug-Report-Photos/raw/master/5j.jpg)
We now expand the anvil menu, and see that we have another anvil, exactly the same!
(pic: https://github.com/TheRoyalBlock/Minecraft-Bug-Report-Photos/raw/master/6j.jpg)

I hope this helped!

P.S: Here's a picture with the build number included, as that seems to have been cropped out in the other images! https://github.com/TheRoyalBlock/Minecraft-Bug-Report-Photos/raw/master/build.jpg

An edit will be added as soon as I get a chance to restart Minecraft (people are still playing on my world.

EDIT: I closed and reopened minecraft, the issue was not fixed. I reinstalled it, and it worked again. Still a valid issue, though.

EDIT 2: After playing for about 25 more minutes, the items disappeared again.