If I have two lines (for example, two bass lines that one of them is inverted one of the other), after I mono the track, (for example listen it on my mobile phone), these two lines will cancel each other, and I wont be able to hear any bass sound. but is that the only side effect? I mean, is there a chance that this anti-phase problem, create some kind of bad distortion or something?
Phase cancellation is a phenomenon that sound engineers usually address during mixing and recording to minimize its impact on normal listening conditions, including mono outputs. It’s mostly a concern for electronic instruments that use samples or synthesis to create sounds with fixed pitches and phase. Acoustic instruments have fewer problems with phase, because the sounds they produce have a much greater variability in phase and pitch even when nominally playing in unison. Instead of phase cancellation, you normally get a “chorus” sound from acoustic instruments.
That said, electronic pickups & microphones can still cause some phase problems even with acoustic instruments, especially if you use stereo microphone configurations. Again, this is a problem that recording engineers test for and resolve, either at performance time or at mixing time. If your final mix ends up phase-canceled, then the recording or mixing engineer made a mistake, presuming it wasn’t done for deliberate artistic effect.
If you mix your own music, and you don’t control for phase, then you will see unpredictable results ranging from weak drum hits to random volume fluctuations to weird filtering effects. You don’t even need to reduce a stereo mix to mono to hear this; simply mixing down multiple mics or tracks into one stereo channel can cause it.
If the two basslines are exactly the same and you invert the phase 180 degrees then yes they will cancel out. But in a lot of real world scenarios you might not be dealing with that theoretical purity. You might have two microphones that are picking up the same thing at slightly different distances for example.
So at times the waves will align and at times they won't causing a mix of adding and subtracting to the combined signal. The auditory effect of that is called "comb filtering" and you can search YouTube to hear what it sounds like.