diff --git a/src/spec.cr b/src/spec.cr
index d47f099..804c528 100644
--- a/src/spec.cr
+++ b/src/spec.cr
@@ -219,25 +219,39 @@ class Specs
 		reg = /%\{([^}]*)\}/
 
 
+		is_missing_references = false
 		value = v.value
 
-		while value =~ reg
+		begin
+			while value =~ reg
 
-			x = reg.match(value)
-			unless x.nil?
-				var = x.captures()
+				x = reg.match(value)
+				unless x.nil?
+					var = x.captures()
 
-				replacement_object = @assignments[var[0]]
-				case replacement_object
-				when StringContainer
-					replacement_value = replacement_object.value
-					value = value.gsub "%{#{var[0]}}", "#{replacement_value}"
+					if var[0]? && @assignments[var[0]]?
+							replacement_object = @assignments[var[0]]
+						case replacement_object
+						when StringContainer
+							replacement_value = replacement_object.value
+							value = value.gsub "%{#{var[0]}}", "#{replacement_value}"
+						end
+					else
+						is_missing_references = true
+						raise "cannot find variable \033[31m#{var[0]}\033[00m"
+					end
 				end
+
+				v.value = value
+
+				value = v.value
 			end
+		rescue e
+			puts "#{e}"
+		end
 
-			v.value = value
-
-			value = v.value
+		if is_missing_references
+			raise "there are missing references in the document: fix it"
 		end
 	end
 
@@ -248,26 +262,41 @@ class Specs
 
 		newarray = Array(String).new
 
+		is_missing_references = false
+
 		str_array.each do |value|
 			tmp = value
-			while tmp =~ reg
+			begin
 
-				x = reg.match(tmp)
-				unless x.nil?
-					var = x.captures()
+				while tmp =~ reg
 
-					replacement_object = @assignments[var[0]]
-					case replacement_object
-					when StringContainer
-						replacement_value = replacement_object.value
-						tmp = tmp.gsub "%{#{var[0]}}", "#{replacement_value}"
+					x = reg.match(tmp)
+					unless x.nil?
+						var = x.captures()
+
+						if var[0]? && @assignments[var[0]]?
+							replacement_object = @assignments[var[0]]
+							case replacement_object
+							when StringContainer
+								replacement_value = replacement_object.value
+								tmp = tmp.gsub "%{#{var[0]}}", "#{replacement_value}"
+							end
+						else
+							is_missing_references = true
+							raise "cannot find variable \033[31m#{var[0]}\033[00m"
+						end
 					end
 				end
+			rescue e
+				puts "#{e}"
 			end
 
 			newarray.push tmp
 		end
 
+		if is_missing_references
+			raise "there are missing references in the document: fix it"
+		end
 		v.value = newarray
 	end
 
@@ -287,11 +316,49 @@ class Specs
 		end
 	end
 
+	def self.has_file_bugs? (content : String)
+		lines = content.split "\n"
+
+		previous_line_started_with_tab = false
+
+		line_count = 1
+
+		has_bugs = false
+
+		# 1. detect lacking "\n" after a block of text
+		lines.each do |line|
+			if previous_line_started_with_tab
+				unless /^$|^\t/.match(line)
+					puts "there is a problem line #{line_count}: lacking a carriage return"
+					return true
+				end
+			end
+
+			line_count += 1
+
+			previous_line_started_with_tab = /^\t/.match(line)
+
+			if line_count - 1 == lines.size
+				if previous_line_started_with_tab
+					puts "there is a problem with end of file: lacking a carriage return"
+					has_bugs = true
+				end
+			end
+		end
+
+		has_bugs
+	end
+
 	def self.parse(file_name : String) : Specs | Nil
 		begin
 			content = File.read(file_name)
 			content = content.rchop
 
+			# XXX: detect simple and known grammar bugs
+			if Specs.has_file_bugs? (content)
+				raise "the file #{file_name} has bugs"
+			end
+
 			specs = Specs.new
 			tree =  Pegasus::Generated.process(content)
 			specs.parse_tree tree