Is one considered sinful if he entered the bathroom with a piece of bread in his hand, having forgotten about it?

There is no sin or blame in merely entering the bathroom with a piece of bread or other food in one's hand, especially if done out of forgetfulness. As for eating in the bathroom, it is not considered chivalrous and is seen as undermining good conduct, as it contradicts the original purpose of the bathroom. Nevertheless, the one who does so does not commit a sin, especially if it was unintentional, as Allah Almighty says: "And there is no blame upon you for that in which you have erred" {Al-Ahzab: 5}.