I was in creative, holding a leash. I had 3 snow golems (they happen to be invincible, because I NBT edited them, but this was the only thing I did externally) . They were tied to a fence, but the leads and its knot became invisible (yes, another reported issue). I broke the fence, and was able to leash 2 of them but not the third. After the world was reloaded, it was fixed mysteriously.

EDIT:I'm in 14w25b and I feel like I'm experiencing a similar issue. I have two of the golems I had previously, and I put leashes on both of them successfully. When I clicked on a Nether brick fence, however, only one leash attached and I was left holding a leash. I swing like I'm placing the leash, but it doesn't actually attach. When I unleash and releash the golem, it still can't be attached. I can also stretch the leash line waaaaaay too far and the golem is not affected. This time, after reloading the world, the leash was attached to the fence.