Trying to use a grid size bigger than 79(?) causes the image to foobar big 
time and the emergency evaluator / revert screen is presented. This one 
died on a grid size of 85 if that's any use. No time to dig around myself.
  Sorry.

Here is the debug log...

Error: ZeroDivide
12 May 2003 10:18:20 pm

VM: Mac OS - Squeak3.5alpha of '7 January 2003' [latest update: #5169] 
Squeak VM 3.5.0b4
Image: Squeak3.5 [latest update: #5180]

SmallInteger(Object)>>error:
	Receiver: 0
	Arguments and temporary variables:
		aString: 	'ZeroDivide'
	Receiver's instance variables:
0
[] in WorldState>>displayWorldSafely:
	Arguments and temporary variables:
		aWorld: 	a PasteUpMorph(1895) [world]
		oldHandler: 	nil
		errCtx: 	PasteUpMorph(Morph)>>drawOn:
		errMorph: 	a PasteUpMorph(1895) [world]
		err: 	'ZeroDivide'
		rcvr: 	0

[] in BlockContext>>ifError:
	Arguments and temporary variables:
		errorHandlerBlock: 	[] in WorldState>>displayWorldSafely:
		lastHandler: 	nil
		val: 	nil
		activeProcess: 	a Process in nil
		errBlockArgs: 	2
		aString: 	nil
		aReceiver: 	nil
		ex: 	a ZeroDivide

ZeroDivide(Exception)>>handlerAction
	Receiver: a ZeroDivide
	Arguments and temporary variables:
		na: 	1
		handler: 	[] in BlockContext>>ifError:
		returnValue: 	nil
	Receiver's instance variables:
		messageText: 	nil
		initialContext: 	SmallInteger>>quo:
		resignalException: 	nil
		handlerContext: 	BlockContext>>on:do:
		tag: 	nil
		activeHandler: 	nil
		dividend: 	0


--- The full stack ---
SmallInteger(Object)>>error:
[] in WorldState>>displayWorldSafely:
[] in BlockContext>>ifError:
ZeroDivide(Exception)>>handlerAction
  - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
ZeroDivide(Exception)>>signal
SmallInteger>>quo:
SmallInteger(Number)>>truncateTo:
Point>>truncateTo:
FormCanvas>>infiniteFillRectangle:fillStyle:
FormCanvas>>fillRectangle:fillStyle:
FormCanvas(Canvas)>>fillRectangle:fillStyle:borderStyle:
PasteUpMorph(Morph)>>drawOn:
PasteUpMorph>>drawOn:
[] in WorldState>>drawWorld:submorphs:invalidAreasOn:
Rectangle>>allAreasOutsideList:startingAt:do:
Rectangle>>allAreasOutsideList:do:
[] in WorldState>>drawWorld:submorphs:invalidAreasOn:
Array(SequenceableCollection)>>do:
WorldState>>drawWorld:submorphs:invalidAreasOn:
[] in WorldState>>displayWorld:submorphs:
FormCanvas>>roundCornersOf:in:during:
FormCanvas(Canvas)>>roundCornersOf:during:
WorldState>>displayWorld:submorphs:
PasteUpMorph>>privateOuterDisplayWorld
PasteUpMorph>>displayWorld
[] in WorldState>>displayWorldSafely:
BlockContext>>on:do:
BlockContext>>ifError:
WorldState>>displayWorldSafely:
WorldState>>doOneCycleNowFor:
WorldState>>doOneCycleFor:
PasteUpMorph>>doOneCycle
[] in Project class>>spawnNewProcess
[] in BlockContext>>newProcess