To afflict or to answer

(ל:י"ד)

כָּל נֵדֶר וְכָל שְׁבֻעַת אִסָּר לְעַנֹּת נָפֶשׁ

Of course, one should not get too distracted with all the מפיק ה's that they miss the important nuance in this פסוק. An erroneous פתח under the ל would change the meaning of the word from that of affliction to that of answering. En garde!

[Also, it is best to stress the דגש חזק in the נו"ן for the same reason as per the anonymous comment below.]

Let your Soul not be Desolate

Although mistakes in the הפטרה might not be as serious, they tend to be more prevalent since the reader of the הפטרה tends to be less of a seasoned lainer (depending on the shul.) Unfortunately, there was one year I heard this done completely wrong every time. In the הפטרה of פרשת פרה, towards the end, there are numerous instances of the word נְּשַׁמָּה, meaning desolate. If it is mispronounced נְּשָׁמָה it refers to the soul. Two words, nearly indistinguishably similar whose meanings couldn't be further apart. It would be advisable, if you know who will be laining the הפטרה, to alert them to this in advance. Fortunately, our shul has acquired קלף's for all the הפטרות and now only actual בעלי קריאה lain it.

As MG points out in the comments - for those who are particular about pronouncing a דגש חזק - the דגש in the מ is also an important distinction between the two words.

קול ענות

(ל"ב:י"ח)
וַיּאמֶר אֵין קוֹל עֲנוֹת גְּבוּרָה וְאֵין קוֹל עֲנוֹת חֲלוּשָׁה קוֹל עַנּוֹת אָנכִי שׁמֵעַ

The pronunciation of the דגש חזק is an art which has fallen largely out of practice. Even amongst the best of בעלי קריאה I have heard few who actually still do. More often the not, the דגש does not change the meaning of the word in and of itself. However, one of my Rebbeim in ישיבת אור ירושלים once pointed out to me that in the above פסוק, the דגש diffrentiates between two words in the very same pasuk!

משה רבינו is answering יהושע that he does not hear the sound of the (victorious) outcry of the mighty, nor the (defeated) outcry of the weak. In those first two instances, the word ענות is from the verb לענות, to answer or to exclaim. Rather, says משה רבינו, it is the call of blasphemy, as רש"י explains, which afflict the souls of those who hear them. Here, the word ענות is from the word ענוי, affliction. Clearly, there are two different words in this pasuk and the only to diffrentiate between the two is with the pronunciation of the דגש.

To Afflict the Corrector

ח:ג וַיְעַנְּךָ

Need I say more? I think I do. Most people with even a slight דקדוק awareness will know that it is important to not pronounce this וַיַעַנְךָ. That would mean "and He answered you," rather than "and He afflicted you." You might hear some בעלי קריאה making the שוא very clear to show that they are saying it correctly. However, this too is incorrect. It is a שוא נח under the יו"ד. So the actual proper pronunciation would be: vay-a-ne-CHA. One has to be very careful to make the "Syllable Stop" before the פתח. I'm not sure if there is a better technical term for that but I'm going with it for now - the art of "pronouncing" the שוא נח such that it breaks the syllable such as in אַתָּה הָרְאֵתָ last week -  har-EI-sa, as opposed to ha-REI-sa.

Of course, the real problem becomes that the very correct pronunciation here is barely discernible from the very incorrect pronunciation, which makes my job all the more difficult. So, maybe pronouncing that שוא נע isn't such a bad idea after all.

As another reader has pointed out, the דגש חזק in the נ is another important differentiating factor. Properly executing it should remove all doubt as to whether the word has been pronounced correctly.

From the Children of the Hebrews

(ב:ו)

וַתֹּאמֶר מִיַּלְדֵי הָעִבְרִים זֶה

It would seem that the pronunciation of the word מילדי is quite crucial. Obviously, when pronounced correctly, it means "from the children of the Hebrews." However, suppose it were to be erroneously pronounced מְיַלְדֵי. Wouldn't that mean "the midwives?" (or, midhusbands, per Michael's comment.) Admittedly, that wouldn't make much sense in context and perhaps need not be corrected. But certainly something to be mindful of.

See the comments.