Inconsistent Watering Practices
Water stress—either drought or waterlogging—can cause blossom drop, misshapen fruit, or even root rot. I once went on vacation and returned to soggy pots of zucchini seedlings, some rotted at the crown!
Aim for deep, consistent soakings: about an inch of water per week, delivered at the base of the plant. Mulch around the stems to conserve moisture and discourage weeds. Your zucchinis will reward you with firm, well-formed fruits rather than bitter or pitted ones.