It's not a bug, it's the bottom row of pixels wrapping back to the top. It's just how computer graphics work, nothing specific to Construct 2 here.
Not in a games context but I've been working with computer graphics / images for about 20 years now and I've never come across that before. I'm assuming you mean just in the context of games graphics and tiling images but I'll take your word for it - it's clearly intentional. <img src="smileys/smiley1.gif" border="0" align="middle" />
The last time I programmed a game and used tiled backgrounds was on the Spectrum 48k iirc, I may have some catching up to do.
The workaround: add an extra row of pixels to the image, or resize the image ever so slightly smaller. For example, make the image 32x33 to add an extra row of pixels along the bottom, and make them the same color. Or, just resize the Tiled Background to be e.g. 64x31, so the last row of pixels doesn't wrap around. Or, just position the other backgrounds to slightly overlap and hide the seams.
OK, just to clarify before I start altering all my images then;
Power of two images are recommended for tiling aren't they so if I use a 32x33 image, won't that occupy the space of a 64x64 one being over the 32x32 size?
If I went a little smaller instead, how does that prevent the bottom row from wrapping - does it only wrap with power of two sizes?
Oh - crash wise, drivers are up to date etc. I think it's always when previewing and then switching back to the C2 window but I'll make notes and bug it.