A commonly used phrase is 没关系 (méiguānxi) which means "it doesn't matter"; it's used respond to 谢谢 (thank you) and 对不起 (sorry).
I found in this question that 系 can also be pronounced jì, as in 系好安全带 (jìhǎo ānquándài) meaning "fasten your seatbelt".
Question: When is 系 pronounced xì/xi and when is it pronounced jì？
Doing a bit of Googling, I find:
- 中文系 = Zhōngwén xì (Chinese department)
- 太阳系 = tài yáng xì (solar system)
- 系统 = xì tǒng (system)
- 联系 = lián xì (to contact)
It looks like xì/xi is the default pronunciation. I didn't find much with jì, except possibly 系好鞋带 (jìhǎo xiédài) = "tie your shoes".