Question Summary:
Mistake someone else for another.

Question Detail:

I was on the streets and met a qadiyani who had a little debate with us. Later on the day I saw some one in the masjid who looked like him so I got confused and thought it was him. I told my friend that this guy is a qadiyani who I met on the streets but I didnt say anything direct to the guy in the masjid. Later on I relised he wasnt the qadiyani guy I had met on the streets. So my question is did I commit a huge sin or will this be foriven cos I made a mistake.

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
The confusion was a mistake. There is no sin for that. However, you should inform your friend of your mistake to avoid him having a wrong thought of the person in the masjid.
And Allah Ta’āla Knows Best
Huzaifah Deedat
Student Darul Iftaa
Lusaka, Zambia
Checked and Approved by,
Mufti Ebrahim Desai.
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الدر المختار - ط. الفكر (6/ 408)
 وإذا كان الرجل يصوم ويصلي ويضر الاس بيده ولسانه فذكره بما فيه ليس بغيبة حتى لو أخبر السلطان بذلك ليزجره لا إثم عليه  وقالوا إن علم أن أباه يقدر على منعه أعلمه ولو بكتابة وإلا لا كي لا تقع العداوة 
 وتمامه في الدرر ( وكذا ) لا إثم عليه ( لو ذكر مساوىء أخيه على وجه الاهتمام لا يكون غيبة إنما الغيبة أن يذكر على وجه الغضب يريد السب