Vertex weilded to XZ plane

I’m running Blender 2.57.0 r36273, and I’ve just added a simple plane to a WIP mesh, and then pressed R X 90, which should have turned it vertical. What it has done though is slanted it (as shown below) and weilded the top two vertexes to the XZ plane. I cannot move them along the y-axis for love nor money; even after restarting the programme and going back to earlier file versions. This is rather frustrating. I’m sure I’ve accidentally turned on some kind of incredably useful feature, but I’d really like to know what it is so I can turn the damn thing off, and never turn it on again.

Can anyone help?

can you loadup sample file

might bve easier to see wht’a wrong !

happy 2.5