I was getting the following error:
 

Msg 50000, Level 16, State 1, Procedure Product_Audit_Update, Line 460
error in [dbo].[Product_audit_update] trigger
Msg 3616, Level 16, State 1, Procedure ProductSave, Line 126
Transaction doomed in trigger. Batch has been aborted.

 
Turns out the following line of code was causing the problem:
 

AND isnull(Inserted.[guidColumn],'') <> isnull(Deleted.[guidColumn],'')

 
As as example:
 

declare @guid uniqueidentifier
declare @guid1 uniqueidentifier
set @guid=newid()

 

if (@guid <> isnull(@guid1,''))
    print 'this works'

 
Causes this error as well:
 

Msg 8169, Level 16, State 2, Line 6
Conversion failed when converting from a character string to uniqueidentifier.

 
I would suggest removing the isnull(,'') and allow the column values to compare by themselves.
 

AND Inserted.[guidColumn] <>Deleted.[guidColumn] 

 
Shane.