The batter wasn’t spread evenly in the pan.
The oven rack wasn’t level.
A pan larger than called for in the recipe was used, causing the batter to be thin and dry.
Oven temperature is too high. Check with an oven thermometer.
Next time, check the bars 5 minutes sooner than the baking time given in the recipe.
A pan smaller than called for in the recipe was used
Stir in dry ingredients with a wooden spoon and don’t overmix, which can develop the gluten in the flour and cause the bars to be tough.
The crust was not baked long enough before placing the filling on top.
Next time, cut in a little more butter so that the crust will stick together.
Cool the bars completely before cutting.
Use a sawing motion when cutting.
Warm the blade of knife in hot water, dry, then cut. Clean and rewarm the knife after each cut.