Henry said:
			
		
	
	
		
		
			Internet Lesson #2: When criticizing grammar or spelling, BE SURE that the criticizing post is without blemish. 
		 
		
	 
I could be wrong, but I think your sentence should read, "(w)hen criticizing grammar or spelling, BE SURE that the 
critical post is without blemish."
The noun in the second part of your sentence is "post" - this is the noun being modified by the derivative of the verb "criticize."  The post itself does not and cannot criticize; rather, the 
writer of the post does so.  Hence, the word "criticizing" must be intended as an adjective - a way of describing the (content of the) post, not the actions the post is taking (it is a mere object and incapable of action in and of itself).
Verbs that describe things that cause an emotion may be tranformed to nouns via the -ing suffix (e.g., satisfying, boring, exciting). "Criticize" does not directly indicate emotion is caused ("satisfy" or "bore" do).  Although the reaction to criticism is often an emotional one, "criticize" itself does not directly indicate a change in emotion.
Verbs that describe an incomplete or continuous action may also be transformed to adjectives via the -ing suffix (e.g., "the falling leaves", "the rushing water" - contrast the difference in meaning between "I saw him climbing the tree" - wherein is implied only that part of the action of climbing was observed, perhaps without seeing the climb completed - with "I saw him climb the tree" - wherein a complete climb, from start to finish, is implied).  
However, in the example above, the criticism is a complete action (and must necessarily be, the post is a complete literary work unto itself; later posts on the same topic do not continue the action of that post).
Thus, the correct adjective in this case - used to describe a piece of literature that is written to criticize - should be "critical" rather than "criticizing." 
--The Sigil
/threadjack