Brachot 47 - Etiquette
One should try to honor others as much as he can, but there are exceptions. On the road one should not allow a more important person to proceed first if this will upset the traffic, nor should one allow him to pass on the bridge first if there is any danger crossing that bridge.
Ravin and Abaye were once traveling on the road, and Ravin's donkey went ahead of Abaye's donkey. Abaye reasoned to himself, "Ever since this Rabbi (Ravin) returned from Israel, he became haughty." When they came to an entrance of a synagogue, Ravin said, "Let the master go in first." Abaye asked him, "And until now I was not the master?" However, Ravin answered by quoting Rabbi Yochanan - "We don't accord honor at the entrance unless it has a mezuzah." But the synagogue does not require a mezuzah!? - Ravin would explain, "Such an entrance that is fit for a mezuzah."
Rav Yehudah said, "People who are dining together are not permitted to eat anything until the one who blesses and breaks the bread tastes of it first." Rav Safra said, "Not permitted to taste ." But there is no difference between them, both teach proper etiquette, only one is supposed to relate the rules he heard from his teacher using the exact words of the teacher.
Art: Willem Kalf - Landscape With A Man Unloading Produce From A Donkey