Is there a sin incurred by combining and shortening prayers during a journey that some consider a journey while others do not, especially if the traveler did not remember their intention before the journey or before the prayer time elapsed?

When a person undertakes a journey that is considered legally valid (safar mu'tabar shar'an), it becomes permissible for him to avail himself of the concessions (rukhsah), even if he did not intend to do so when he set out. A common person (al-'ammi) does not incur sin if he imitates (follows) someone whose knowledge and piety among the people of knowledge are trusted. Therefore, if you act upon the opinion of Sheikh al-Islam and those who agreed with him on this matter, there is no sin upon you regarding shortening prayers (qasr) or combining them (jam'), as long as you are following one of the trustworthy scholars.