A classic example of carelessness is not returning borrowed items. It seems to be an all-too-common occurrence that people lend sefarim out and never see them again! Unless the lender intends to forgive failure to return the sefer, this constitutes a form of gezel. Of course people do not purposely intend to steal, but such negligence surely stems from a lack of respect for other people’s property. Rav Yaakov Kamenetsky zt”l was a living example of how to act in this area. On one occasion he was filling in a kesubah and used the chassan’s pen and forgot to return it to him amidst the hectic nature of the wedding. TWO YEARS later he met again with the pen owner and handed him the pen .
Another area in which there is a great yetser hara to be moreh heter is using other people’s items without express permission. There are many instances in which it is forbidden to assume that the owner will be mochel for someone to use his item without asking first. The ease at which one can be nichshal in this area is demonstrated in the following story. Rav Leib Chassid was the famous tzaddik of Kelm. In his later years he went out for a walk on the road between Kelm and Tavrig. One day a teenage boy driving a wagon passed by and offered him a ride. Reb Leib asked him if the wagon was his, and the by replied that it belonged to his father. “Did he give you permission to take passengers?” Reb Leib asked. The boy admitted that he had never discussed it with his father, adding, “Do I really need his permission for that?” “Yes”, said Reb Leib, “since you have not asked permission you would be a thief if you took any passengers into the wagon. ” It is such sensitivity that is required in order to avoid erring in these halachos.
Excerpted from here.