Did you use bottled lemon juice or real lemons? Did you add more lemon juice than is called for? Too much acid from the lemon juice can breakdown the cornstarch in the pudding. Use bottled lemon juice because the acid tends to be about the same from bottle-to-bottle (which is why it is always recommended over real lemons in home canning). The acidity in real lemons can be all over the place. Both too much heat (did your pie sit in a hot place in a hot kitchen at any time) and too much acid will destroy the thickening ability of the cornstarch in the pudding.

-Grainlady

That's true, bottled lemon juice is consistent in acidity but it doesn't taste as good, so there's a trade off. I use fresh.

I wouldn't think freezing would affect the lemon filling as much as it would the meringue, but I've never frozen one.

Perhaps a different thickener would hold up better I'm assuming this is an anomaly, something that doesn't normally happen and that you have a favorite recipe you usually use?

Annie

I use the bottled juice for pie and save the Good Juice (which I get when a friend sends me Meyer Lemons from her tree every year) for things where it makes a noticeable difference. Which is not in a pie. The bottled just gives better results when thickening with cornstarch, which I GREATLY prefer over flour.

On the other hand you could try some alternative thickeners such as arrowroot, or yucca/cassava/tapioca flour/starch (they're all really the same thing - excepting the arrowroot, that's unrelated). Fresh lemons vary way too much in acidity to risk using cornstarch as thickener - you just can't tell how much cornstarch you will need, wherease since the bottled stuff is stabilized, you can always rely on being able to use the same amount of cornstarch each and every time. I don't know about the susceptibility of either of the above to acidity-caused breakdown, but anything is better than having to resort to flour ...

I just checked, and arrowroot should be great for lemon meringue. It isn't affected by the acidity of the lemons as cornstarch is.

Here is a link that might be useful: Thickeners Revealed

It takes twice as much flour to substitute for cornstarch, which could result in "lemon cake", instead of pudding. And there are recipes that use both cornstarch and flour in the recipe.

The amount of cornstarch, lemon juice, and sugar used in a recipe also varies, which can make one recipe much different from another. One may be best consumed the same day, another a better "keeper". Here are two recipes that are "miles" apart in ingredient amounts.

King Arthur - Classic Lemon Meringue Pie:

1 1/2 cups (10 1/2 ounces) sugar
6 tablespoons (1 7/8 ounces) cornstarch
1 1/2 cups cold water
3 large eggs yolks (reserve the whites)
3 tablespoons (1 1/2 ounces) butter
Grated peel of 1 large lemon
1/2 cup (4 ounces) lemon juice (juice of about 2 lemons)
1/2 teaspoon vanilla extract