Why izzit...?

Donna, this may be caused by a number of reasons.

The most common one is the Kaleidoscope component itself, especially when used in a combo with Perlin Noise or any other noise -- when Seamless is turned on, the noise behind the caleidoscope converts itself into a seamless version, which is different from the non-seamless version (especially on the edges). If the Kaleidoscope's pattern uses regions close to the noise's edges, this will cause the final pattern to change when Seamless is turned on.

In this particular case (Caleidoscopy) the problem is caused by the fact that the Offset component behaves differently when Seamless Tiling is turned on. The filter uses Offset as an elevation gradient, and when you turn Seamless Tiling on, the changes in the gradient lead to the changes in the final image.