In פרשת שופטים, there are three instance where the term דם נקי, or a variation thereof, appears. If the vowel underneath the ד is a פתח, it would indicate סמיכות and the translation would be, "the blood of a clean (innocent) person." If it were a קמץ, it would simply mean clean blood. Although one might argue that the connotation is ultimately the same, I would say this is still a rather significant difference. Oh, wouldn't it be nice and simple if it were always the same. Alas, not only is it not always the same, there is not always 100% clarity as to which way it should be pronounced.
In שופטים, we have the following:
י"ט:י וְלֹא יִשָּׁפֵךְ דָּם נָקִי
י"ט:י"ג וּבִעַרְתָּ דַם הַנָּקִי
כ"א:ח וְאַל-תִּתֵּן דָּם נָקִי
The latter two appear to be undisputed. However, someone came over to me about the first one suggesting the בעל קריאה had said it wrong. I was certain he had not. But sure enough his (Artscroll) חומש had a פתח while mine (חורב) had a קמץ. I have embedded a snippet from the ספר אם למקרא למסורת below which should bring some clarity to the subject.