THE FOLLOWING ARE ALL GRAMMATICALLY CORRECT.
+"Buffalo buffalo Buffalo buffalo buffalo buffalo Buffalo buffalo"
+"Colorless green ideas sleep furiously"
+James, while John had had "had", had had "had had"; "had had" had had a better effect on the teacher.
+"Time flies like an arrow; fruit flies like a banana"
+That that exists exists in that that that that exists exists in.
+ "That that is, is. That that is not, is not. Is that it? It is."
+If it is it, it is it. If it is, it is it, it is!
+That that exists exists in that that that that exists exists in