Ahhh, crosstalk - it’s there in just about every system. A real hot (clipping level) signal on the mic input or instrument input can show up as a slight blip (-65dB or lower) on the Line Input metering (and vice versa). If you’re using the line inputs, you probably want to disconnect anything from the front inputs while recording. BTW, according to Wikipedia, Crosstalk is also an English translation of the Chinese term xiangsheng, meaning a traditional bantering-style dialogue between two comedians rich in puns and allusions. Its closest equivalent in the English-speaking world would probably be the "Who's on First?" routine of Abbott and Costello.