''What comes to you of good is from Allah, but what comes to you of evil, [O man], is from yourself'' (4:79)
A man leaves his house, on the way a black car crosses his path, he thinks this is a bad omen and therefore returns back home. Though this seems to be such a minor act, however in reality, it really mean that he thinks that the black cat has power and authority to change the man's fate. This is shirk because it mean that other then Allah the car also has control and authority over a man's life and to bring harm to him (Naauuzobillah).
Other common Superstitious Beliefs prevalent in our society are seeing someone disliked in the morning means the whole day will be full of bad luck, twitching eyes means bad luck and evil portent, Itching in tthe rigth palm means money will be acquired. If one shoes comes on top of the other, it means journey will be undertaken. Taking omens from colors, such as wearing black means illness will strike, or not wearing black clothes because black is the color of the cover of kaabah, not wearing green colored shoes due to respect because green is the color of the dome of Prophet's (s) Mosque.
Holding such beliefs not only led man to make halal things haram on himself but led him to associate fate and destiny with these things, which is not only great misguidance but a great sin "shirk" because no one other than Allah has the power and control over man's fate. and all the things from which good or bad portents are derived are nothing but creations of Allah who have no control over other creations.
Prophet Muhammad (peace be upon him) said,
"taking omens is shirk; taking omens is shirk. He said it three times. Every one of us has some (superstition), but Allah removes it by trust (in Him)." (sunan abi dawood)
He also said, "A person who derived bad luck from something and refrained from doing what he was intending to do, he has committed shirk. The companions asked him whar would be its expiation, the Prophet replied "He should say
''O Allah this is no portent other then Your portent, no goodness other than Your goodness, and none worth of worship other than you'."(Musnad Ahmed)