What would happen if blades are not pitched correctly on a propeller blades?
For a start, whoever built the propeller would get investigated for possible drug/alcohol impairment on the job.

In terms of trying to run the propeller, it would generate a lot of noise (from turbulence of the blades running in "reverse pitch", possibly stalled relative to the airflow induced by the previous blade) and very little or no thrust.  If it's on a piston engine (or electric motor, assuming a model aircraft or drone case), the engine/motor is likely to overheat after a short time due to lack of cooling airflow.

Finally, it's possible vibration induced by the noise-causing turbulence could lead to a structural failure of either the propeller itself, the engine/motor, or the engine/motor mounts.