During an analysis I was performing of the poems contained in the 古今集 I noticed a curious pattern. Otherwise perfectly-formed poems included in the collection seemed to ignore the morae-count on a consistent basis with respect to the verb 思ふ【おもふ】, allowing it an extra beat, as you can see in the following examples:
These examples are not exhaustive; they are merely to demonstrate the phenomenon and to show that it doesn't matter which line it appears in.
What I'm wondering is if the rules on morae-counts are more complex than originally thought and actually have a provision allowing for this, or if there were some other explanation for it.
On a similar note, I noticed while researching this question that あらば gets a pass when on a 5-morae line when paired with 3-kana words (e.g. 「こころあらば」、「いのちあらば」), while the related あれば seems to get a free pass in more settings (about 1/3 of total occurrences). I'm almost inclined to think any potential rule involves the ば form, however that's not a consistent element in the 思ふ cases.