The word yitzok (יִצֹק), meaning he shall pour, which appears in Leviticus 14:26 and Numbers 5:15, should, by the rules of biblical grammar have a dagesh forte in the second letter (צ) because the previous letter is vocalised with a short vowel, namely chirik. But in both cases the dagesh is absent. Why?
I suspect that it's something like this: The root begins with a yod, and the word also deserves a yod with chirik ("yi") prefix for future tense, masculine. So really, it should be written יִיצֹק. For whatever reason, the second yod isn't printed explicitly, but we act as if it's there, making the initial syllable's vowel a chirik gadol, which allows that syllable to stand on its own without the closure that a dagesh chazak in the next letter would afford.
One suggested reason for this omission is found in Joüon/Muraoka's grammar, where they write (section 7c): "Often, as a measure of economy or aesthetics, the mater lectionis is omitted when a ו or י occurs in the same word. Thus גּוֹיִם "peoples" is almost always written for גּוֹיִים, ..."
Here's another example of the same form: The root ישר appears around Tanach in future/perfect, masculine, sometimes with a second yod, sometimes without, in each case without a dagesh in the shin:
- Num. 23:27 יִישַׁר
- I Sam. 18:20 וַיִּשַׁר
- I Sam. 18:26 וַיִּשַׁר
- II Sam. 17:4 וַיִּישַׁר
- II Chron. 30:4 וַיִּישַׁר
Further examples: וי(י)צר in Gen. 2:7 vs. 2:19; וי(י)רא in I Sam. 12:18 vs 18:12; וי(י)שר in I Sam. 18:20 vs. II Sam. 17:4; וי(י)קץ in Gen. 41:4 vs. I Kings 3:15; וי(י)בש in I Kings 17:7 vs. Psalms 102:5; י(י)רש in Gen. 22:17 vs. 24:60; י(י)טב in Gen. 12:13 vs. I Sam 24:5.
Hat-tip to magicker72 for the J/M reference and many examples.