Assertion Error trying to preview

When trying to preview in Firefox 7, get the following:

Assertion failure: Error separating convex polygons for physics collision polygon (1)

Stack trace:

assert2(false,"Error separating convex polygons for physics collision polygon (1)")@http://localhost:50000/preview_prelude.js:12

([object Array])@http://localhost:50000/Physics_behavior.js:625

([object Array],168849)@http://localhost:50000/Physics_behavior.js:519

()@http://localhost:50000/Physics_behavior.js:890

()@http://localhost:50000/Physics_behavior.js:807

([object Array],[object Object],true)@http://localhost:50000/preview.js:840

()@http://localhost:50000/preview.js:1194

()@http://localhost:50000/preview.js:1006

()@http://localhost:50000/preview.js:420

()@http://localhost:50000/preview.js:375

([object HTMLCanvasElement])@http://localhost:50000/preview.js:93

("c2canvas")@http://localhost:50000/preview.js:10

((function (a, b) {return new e.fn.init(a, b, h);}))@http://localhost:50000/preview.html:84

([object HTMLDocument],[object Array])@http://localhost:50000/jquery-1.6.3.min.js:2

()@http://localhost:50000/jquery-1.6.3.min.js:2

([object Event])@http://localhost:50000/jquery-1.6.3.min.js:2

Also, Firebug shows and error in Physics_behaviour.js:

htv is undefined at line 635.

.capx is HERE

I get a similar error now. It says Assertion Failure: Physics polygon separation resulted in no polys. I have no idea what it is. I have narrowed it down to one object on the screen that does this.