Storing:
The custard can be made up to 3 days ahead, if desired. The assembled pie will keep in the refrigerator for up to a week.
If Custard Turns Out Too Loose:
Put custard back on the stove and heat on medium heat, following the same sequence you did previously (but probably for a little longer this time around). Be sure to keep on the heat until it boils, whisking constantly. Keep whisking 1-2 minutes while boiling. Then refrigerate again to cool.
If Custard Turns Out Too Thick:
Put custard back on the stove and add about 1/2 cup of milk on medium heat to loosen it up. Then add 6 very generous tablespoons (cereal spoons, let's be honest here) of whipped cream and whip it with the hand mixer (normal beaters) to fluff it up.
Adapted from NYT The Baker's Apprentice: Banana Cream Pie