It's because the error occurs in the critical section of AccessLock, so 
it'll stay locked during error handling. This probably locks up the 
debugger, because it can't access the preferences.
Here's a fix:

preferenceAt: aSymbol ifAbsent: aBlock
 	"Answer the Preference object at the given symbol, or the value of aBlock if not present"

 	self accessDictionaryOfPreferencesIn: [ :dictionaryOfPreferences |
 		dictionaryOfPreferences
 			at: aSymbol
 			ifPresent: [ :preference | ^preference ] ].
 	^aBlock value

Levente

On Fri, 9 Jan 2015, karl ramberg wrote:

> Hi,
> Image lock up doing
> Preferences togglePreference: #justATest
> 
> If I change error:  to inform: in the method I do not get a image lock up.
> 
> This is in a trunk image and tested with several Cog VM on Windows.
> 
> Preferences>>togglePreference: prefSymbol
> "Toggle the given preference. prefSymbol must be of a boolean preference"
> (self preferenceAt: prefSymbol ifAbsent: [self error: 'unknown preference: ', prefSymbol]) togglePreferenceValue 
> 
> Karl
> 
>