If a block shorter than a slab (snow, carpet, etc.) is on top of a full block, the player will not auto-jump until the extra block height is removed. Only texted with pressure plates, which seem to be fine, carpets, which are glitchy, and snow layers, which are also glitchy.
Environment
Windows 7 Desktop Computer, 8 GB RAM, no mods or third-party software, singleplayer.
I believe that carpet should prevent auto-jump, but not one layer of snow, because it doesn't change your height.