On Silversea I was sent away from the main dining room TWICE, first time it was an informal night, I had had a nice shirt but no jacket wich was mandatory
The second time I slipped in with a group of people, I had a shirt AND jacket but no tie (formal night)
they brought me a brown tie (with my blue jacket very nice??)
I have taken 50+ cruise (4 on Crystal) but I have never had any problems with dress code