In the newest version of mirc there is a bug - if one tells mirc to warn the user when he tries to paste MORE than ONE line, mirc asks for confirmation even if I try to paste just one line! what's even worse, mirc overrides all integrated scripts, I have a script installed called paste enhancements, it checks for pastes, and warns if more than one line is pasted, but it's totally useless if this function is also checked in the mirc settings. so I hope this bug will be fixed, till then I suggest you switch off this function in mirc and install the paste script, which works flawlessly and only asks for confirmation if indeed more than one line is pasted
