what would happen if you went through the whole cloning process but started with a cancerous cell? think about it

That might depend on the type of error. But I'm going with failed clone.

"Immortal" cell lines could be said to be cancerous, and they're extremely useful, but if you want to clone a whole organism from a cancer...

You are probably right. The malfunctioning apoptosis in cancer cells could be one reason. There would be no way to nicely get rid of defect stemcells.

Also I think the cancer cell is only geared towards multiplying, so it probably won't respond properly to the cell signals involved in growing an embryo.

-Splicer

It wouldn't work. Cancer is basically a disease where developmental programs are expressed in cells uncontrollably. If you'd want to clone a healthy animal, you would definaltely need normal genome as a starting material for the developmental programs to work properly.

Besides, for a cancer you'll need more than than just one mutation. First you'd need a mutation in the apoptosis cascade to stop the cell from killing itself. Second you'd need a mutation to increase the proliferation of the cell, either by over expressing oncogenes or by silencing tumor suppressor genes. Next you'd propably have a mutation in the DNA repair machinery to facilitate accumulation of further mutations. Fourth would propably be some kind of metabolic mutation to allow tuomors to grow under low oxygen conditions (the diffusion of oxygen would limit the size of tumors, see Wartburg effect). And finally, if cancer is considered to be malignant, you'd need some mutation to break the tissue integrity.

So, to sum it all up: It wouldn't work, and you wouldn't be getting any monsterous mutant creatures or zombies or anything

cancer cells are very popular in biotech because of their immortality.
The Hybridoma technology (a way to produce monoclonal antibodies) for example includes fusing a specific antibody-producing B cell with a myeloma and cloning it.