I was already aware of things like seppuku and honor however I didn’t think it was this severe, one mistake in terms of politeness and someone is either executed or asks to commit seppuku, was it really this severe or is it an over exaggeration for sake of tension and shock?
The novel comes up in questions from time to time. While I have not read it or watched the TV series, from descriptions of it and from past answers it seems greatly exaggerate these kind of exotic things for drama and world building.
Specifically in this case, no, people didn't commit seppuku at the drop of a proverbial hat. The two answers by /u/NientedeNada found here specifically deal with the question at hand. You might be also interested on our FAQ on seppuku.