Don't fully know how to describe this, so see the images below.
If you have a dispenser without a block below the face, it will use the durability of Flint & Steel, but it doesn't actually light a fire. It should be able to light the side of itself/a block in front of it, but it doesn't. It only uses the durability.
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Just tested, and you're right. It seems to only work with flammable materials, which to me makes no sense. Maybe Mojang should do something about that? (I know the mods are gonna think this is a suggestion, which it's not. I actually don't think it should be this way.)
Edit: Also, the issue still remains of it using durability when it shouldn't.
Both are confirmed.
I experienced it in 13w10a too
Not sure if this is the right place to post, but flint and steel will not ignite any block on the Xbox version. Just spits it out.
This site is for bug reports for the PC version of Minecraft, for the Xbox 360 version please go here.

Confirmed for 14w21b

How is this different from MC-4923?
This is with a dispenser and works even if the wall material is inflammable.

Confirmed for 14w28a
But Torabi is somehow correct, it seems like the dispenser is also creating a "wrong" fire which gets removed instantly (you can test this by placing a BUD there)
So this is also a part of MC-4923 and also the bug it is really describing is that you can't put Netherrack at the sides on fire
The same happens if you try to do it yourself, try to light the side of the netherrack for the instance, so Works as intended.