10 common mistakes in Prayer (Salah, Salat or Namaz):

1. Delaying the Salah intentionally: While being busy in worldly affairs we often delay the Salah from its preferred time. This is the most common mistake in Prayer made by most of us. We think that Praying is last thing to do and we must attend to other matters first. Many workers delay Zuhr and Asr Prayer till night-time. Quran speaks strictly on guarding our Salat:

╚► ï¿½Guard strictly (five obligatory) As-Salawat (the prayers) especially the middle Salat (i.e. Asr Prayers)� [2:238]Here to safeguard the Salat means to perform it at its prescribed time and to be steadfast upon it.

╚►Narrated Ibn Umar (R.A): Allah�s Messenger (ﷺ) said: �Whoever misses the Asr prayers (intentionally), then it is as if he lost his family and property.� [Al-Bukhari]
Another Hadith explains the importance of praying on time:
╚►The Messenger of Allah ﷺ said, (This is the prayer of the hypocrite, this is the prayer of the hypocrite, this is the prayer of the hypocrite. He sits watching the sun until it is between the two horns of Shaytan. Then he stands and pecks four (Rak`ahs) and he does not remember Allah (in them) except very little.) This Hadith is describing the end of the time for the `Asr prayer, which is the middle prayer as is confirmed by a text (Hadith). This is the time in which it is disliked to pray. Then this person stands to pray it, pecking in it like the pecking of a crow. He does not have tranquility or humility in it at all. 
Thus, the Prophet ﷺ said, 
(He does not remember Allah (in them) except very little.) He probably only stands to pray it so that the people will see him praying, and not seeking the Face of Allah. This is just as if he did not pray at all. 

╚►Allah says, 
So woe to the worshippers, who are neglectful of their prayers, Those who (want but) to be seen (of men), [Al-Maa�un 107:4-6]Verily, the hypocrites seek to deceive Allah, but it is He Who deceives them. And when they stand up with laziness and to be seen of men, and they do not remember Allah but little. [4:142]2. Reciting Holy Quran fast in the Prayers: we must pause and take breaks while reciting Holy Quran during Salat. We should not recite the Surah�s so fast that that the verse get intermingled. One of the Hadith emphasizes this issue:╚► ï¿½The Prophet (ﷺ) used to pause after each verse of Surah al-Fatiha. [Abu Dawood]
╚►The Prophet (ﷺ), who said : 
"A prayer performed by someone who has not recited the Essence of the Quran (Surat al-Fatihah) during it is deficient (and he repeated the word three times), incomplete. Someone said to Abu Hurayrah: [Even though] we are behind the imam? He said: Recite it to yourself, for I have heard the Prophet (ﷺ) say: Allah (mighty and sublime be He), had said: I have divided prayer between Myself and My servant into two halves, and My servant shall have what he has asked for. 
When the servant says: Al-hamdu lillahi rabbi l-alamin ("Praise be to Allah, Lord of the Worlds."), 
Allah says: My servant has praised Me. 
And when he says: Ar-rahmani r-rahim ("The Merciful, the Compassionate"), 
Allah says: My servant has extolled Me, 
and when he says: Maliki yawmi d-din ("Master of the Day of Judgement"), 
Allah says: My servant has glorified Me - and on one occasion He said: My servant has submitted to My power. 
And when he says: Iyyaka na budu wa iyyaka nasta in ("It is You we worship and it is You we ask for help"), 
He says: This is between Me and My servant, and My servant shall have what he has asked for. And when he says: Ihdina s-sirata l- mustaqim, siratal ladhina an amta alayhim ghayril-maghdubi alayhim wa la d-dallin ("Guide us to the straight path, the path of those upon whom You have bestowed favors, not of those against whom You are angry, nor of those who are astray"), 
He says: This is for My servant, and My servant shall have what he has asked for." [Muslim]

3. Moving before the Imam in congregational prayers: one should not race with the imam, when Imam says Allahu Akbar, then the person who�s following the Imam in Prayers should say Allahu Akbar. Not before Imam or with Imam. The Prophet (ﷺ) states:╚► ï¿½Surely the imam is there to be followed� [Muslim]
Another Hadith states:╚►The Prophet (ﷺ) said: �Does not the one who raises his head before the Imam does fear that Allah would transform his head into a donkey�s head� [Muslim]

4. Performing the Salah hastily: Salat should be performed slowly and calmly. Rukoo or Sujood must be performed unhurriedly. In order to complete the Rukoo one must stay in Rukoo posture long enough to recite �Subhan Rabbi yal Adtheem� at least three times. In the same manner, for the Sujood the person should say �Subhan-na Rabbi yal-ala� three times slowly. This Hadith commands to complete the Rukoo and Sujood in proper manner:
╚►The Messenger of Allah (ﷺ) said: ï¿½The worst thief is the one who steals from his own prayer.� People asked: �Messenger of Allah! How could one steal from his own prayer?� He said: �By not completing its rukoo and sujood� [at-Tabarani]

╚►The Prophet (P.B.U.H) said: �He who does not complete his rukoo and sujood, his prayer is void� [Abu Dawood]
5. Gazing right, left or upward during Salat: While performing Salah, one should concentrate on looking to the point where the head is placed during Sujood rather than looking upwards. Because this may lose concentration. In the Hadith recorded by Sahih Muslim the Prophet (P.B.U.H) said:╚► �Let those who raise their gaze up during prayer stop doing so, or else their sights would not return to them� (i.e. they will lose their eyesight) [Muslim]

6. Performing Sujood improperly: most common mistake observed in Prayers is to perform Sujood by resting only the tip of head on floor.╚►The Prophet (ﷺ) said: �I am commanded to prostrate on seven bones the forehead and the nose, the two hands [palms], the two knees, and the two feet� [Muslim]
7. Going to Mosque for performing Salah with rapid walk:
╚► The Messenger of Allah (ﷺ) said: ï¿½When prayer has already started, do not walk quickly to join in. Proceed calmly and reposefully; then join in whichever part you catch up, and complete whichever part you missed� [Bukhari and Muslim]
8. Eating food of bad smell before performing Prayers: ╚►The Prophet (ﷺ) said: �He who eats from the smelly plant [garlic or onion], let him come not near our mosque; the angels are bothered by that which bothers men� [Muslim] 

9. Not performing Prayers during illness or while traveling: The Prophet (ﷺ) always used to perform Prayers even during His last days before death. He (ﷺ) never neglected it even during combats. Prayer is obligatory and must be performed in any condition. If a person cannot perform ablution due to his illness then he may perform tayammum (dry ablution). If he cannot stand in prayer, he may perform Salah while sitting or even laying down. Same is the case while traveling. The person can combine 2 Salah while traveling i.e. Zuhr Prayer and Asr Prayer. or Maghrib Prayer and Isha Prayer.
10. Crossing in front of a person who is performing Salah: ╚►The Prophet (ﷺ) said: ï¿½Were the one who crosses in front of a praying person to know the consequences of doing so, he would have waited for forty better than to cross in front of him� [Bukhari]
May Allah guide us to perform our Salah perfectly and help us to guard our Salat so when on the Day of Judgment we are questioned about our prayers, we may succeed. Ameen.