For more information on how to perfect your prayer please read pages 110 - 155 in the book 'Laws of Salah' (download here http://bit.ly/1mK8O7N )
Ibn Abideen mentions that a prayer performed with a makrooh tahriman act is wajib (necessary) to repeat and only recommended (to repeat) if the makrooh is tanzihan (minor dislike).
Common mistake #1
It is makruh tahriman (close to haram) to fold your trousers specifically for Salah. You should perform the prayer without folding them.
If the trousers are below the ankles and you have no pride then this would be makruh tanzihan (minor dislike). And you would not need to repeat the prayer. If you have pride wearing long garments this is haram and the prayer would need to be repeated.
If you trousers are folded all the time outside of prayer too, then it will not be makruh tahriman to pray in them in that state.
As for the Prophet's saying: "the one who makes his garment hang down on the ground" then its meaning is: The one who lets it down and drags its extremity OUT OF ARROGANCE (KHAYLA')... Imam Nawawwi
Common mistake #2
From 'Fiqh of mobile phones' - Mufti Monawwar Atiq
Turn off your mobile phone BEFORE entering the Masjid or put it on silent as your calls will distract you and other people in prayer. Do not even have the vibration on as this will lead to distraction in your own prayer if not in others prayers and the essence of humility will be lost.
If the mobile, in any rare case, does ring then one is permitted to enter one hand into the pocket and switch it off. This can be done up to three times in one whole act of prayer e.g. ruku, the third time ones salah will become invalid. If the phone is locked or cannot be silenced without looking at the button then with minimal movement as possible one can look at the mobile to turn it off. Any excessive movement in prayer i.e. that which onlookers look at from far and do not have doubt that you are not in prayer (they immediately think you are not praying due to your movement) invalidates the prayer (Hashiya Ibn Abideen).
If someone praying salah cannot switch off a ringing mobile by minimal movement then he should exit prayer, switch off the mobile and start again just as one is to exit prayer if he needs to go to the toilet because humility is lost by distraction (ibid).
Common mistake #3
Please read http://www.seekerspath.co.uk/question-bank/prayer-dar-al-ifta/q-id0152-sitr-becoming-uncovered-in-prayer/
Sisters should note that praying in see-through clothes/scarves will also make your prayer invalid. The awrah for them is the full body except face, hands & feet.
Common mistake #4
It is obligatory to start the prayer saying 'Allahu Akbar', this must be recited for the prayer to start. This is for both praying alone or praying in a congregation.
Any prayers without the takbir e tahrima (opening takbeer) are invalid.
Common mistake #5
It is obligatory to recite the Qur'an in the prayer when praying alone. It is not sufficient to be thinking of the verses in your mind, they must be uttered and you should be able to hear yourself. Not doing this will invalidate the prayer because qiraat is a compulsory pillar of the prayer.
It is wajib (necessary) to remain silent (for surah fatiha & the other surah) when praying behind an Imam, his recitation suffices for all those behind him (thana, tasbih, tashahud & durood is recited as normal). To recite the Qur'an when praying behind an Imam is makruh tahriman.
Common mistake #6
Self-explanatory. See page 146 of Laws of Salah.
Common mistake #7
Scratching thrice in one rukn (of salah) will invalidate salah i.e. scratching once and then lifting hand, then scratching and lifting hand once again; it is twice-scratching. If the act of scratching is repeated for the third time, salah will become invalid. Placing the hand once (at an organ) and moving it several times will be considered scratching once only.
Common mistake #8
If an adult laughed in a Salah, consisting of Ruku' and Sujud, so loudly that people in vicinity heard his laughing, his Wudu and Salah, both became invalid. If he laughed loudly such that only he listened to it, his Salah will become null (invalid) but there will be no effect on his Wudu. Smiling (without creating a sound) will not nullify Wudu and Salah. (Maraqil Falah, pp. 91)
Common mistake #9
Common mistake #10
Folding cloth, as these days, some people lift their trousers etc. from front or rear side while going down for Sajdah is makruh tahriman. However, if the cloth sticks to the body, there is no harm in un-sticking it with one hand. [Laws of Salah]
Common mistake #11 [MEN ONLY]
To bend all five on each foot is Sunnah. [Bahar e Shariat p513 & p530]
Women do not bend their toes - instead they bend both feet towards the right.