In my local area prabachan guru said pronouncing the mantra's wrongly will not give the desired result and may have bad outcome, also giving one example of that case but I am not able to recall that example maybe from Shrimad Bhagavad Gita (श्रीमद्भगवद्गीता) not sure though.
From the answer of user Swami Vishwananda on this question:
Mantras are based on sound (vibration). If the mantra is pronounced correctly it will create a certain vibration and that vibration sets in motion actions that deliver a certain result.
All mantras must be pronounced correctly to deliver the correct result. If not pronounced correctly, some mantras will not deliver any result. Some mantras if not pronounced correctly will deliver harmful results.
So, I'm interested in knowing some examples of pronouncing the mantra's wrongly from Vedic text with effect (outcome). If it's making question too broad limit your answer to Bhagavad or any famous vedic text.