This is with Ian's VM (3.2.3-devel), under Linux.
This happens occasionally (though most of the time it doesn't).
And I just started seeing it a couple of days ago.

Any ideas?

24 June 2002 8:31:35 am

VM: unix - Squeak3.3alpha of 30 January 2002 [latest update: #4881]
Image: Squeak3.2gamma [latest update: #4889]

CursorWithMask(Object)>>error:
	Receiver: ((CursorWithMask
	extent: 16 at 16
	depth: 1
	fromArray: #(
		2r1100000000000000000000000000
...etc...
	Arguments and temporary variables: 
		aString: 	'a primitive has failed'
	Receiver's instance variables: 
		bits: 	a Bitmap of length 16
		width: 	16
		height: 	16
		depth: 	1
		offset: 	-5 at 0
		maskForm: 	Form(16x16x1)

CursorWithMask(Object)>>primitiveFailed
	Receiver: ((CursorWithMask
	extent: 16 at 16
	depth: 1
	fromArray: #(
		2r1100000000000000000000000000
...etc...
	Arguments and temporary variables: 

	Receiver's instance variables: 
		bits: 	a Bitmap of length 16
		width: 	16
		height: 	16
		depth: 	1
		offset: 	-5 at 0
		maskForm: 	Form(16x16x1)

CursorWithMask(Cursor)>>beCursorWithMask:
	Receiver: ((CursorWithMask
	extent: 16 at 16
	depth: 1
	fromArray: #(
		2r1100000000000000000000000000
...etc...
	Arguments and temporary variables: 
		maskForm: 	Form(16x16x1)
	Receiver's instance variables: 
		bits: 	a Bitmap of length 16
		width: 	16
		height: 	16
		depth: 	1
		offset: 	-5 at 0
		maskForm: 	Form(16x16x1)

CursorWithMask>>beCursor
	Receiver: ((CursorWithMask
	extent: 16 at 16
	depth: 1
	fromArray: #(
		2r1100000000000000000000000000
...etc...
	Arguments and temporary variables: 

	Receiver's instance variables: 
		bits: 	a Bitmap of length 16
		width: 	16
		height: 	16
		depth: 	1
		offset: 	-5 at 0
		maskForm: 	Form(16x16x1)

--- The rest of the stack ---
Cursor class>>currentCursor:
EventSensor(InputSensor)>>currentCursor:
CursorWithMask(Cursor)>>show
NCConnectorMorph>>monitorDestinationForEnd:
StepMessage(MessageSend)>>value
StepMessage(MorphicAlarm)>>value:
WorldState>>runLocalStepMethodsIn:
WorldState>>runStepMethodsIn:
PasteUpMorph>>runStepMethods
WorldState>>doOneCycleNowFor:
WorldState>>doOneCycleFor:
PasteUpMorph>>doOneCycle
[] in Project class>>spawnNewProcess
[] in BlockContext>>newProcess