To cross the line is an idiom that means to behave in a way that is not socially or professionally acceptable. If you cross the line you do something that you know is wrong or will not be received well by others. Similar in meaning to step out of line or overstep the mark.

1. My behaviour at the party was quite boisterous, but I don't think that I crossed the line at any point.
2. You have made numerous insulting comments towards me, but this time you have really crossed the line. I'm not putting up with it anymore.

1. When I was letting Kevin know what he had done wrong I wanted to be firm, but made sure that I didn't cross the line with anything that I said.
2. I can understand the company making cutbacks, but unceremoniously firing members of staff who had been there for over a decade was really crossing the line.