In the aspects of square or semisquare Mercuries...and in the aspect of quincunx and sometimes opposite mercuries...
(not by sign alone though, but giving utmost attention to orb.)
And, (at least for me) when their Mercury squares either my Sun or Moon.
But each of these has a different effect that leads to miscommunication of some sort.
When I encounter someone with squared Mercury to my own, it's not often such a bad thing. For the most part I have to agree with what VenusDiSirius has stated in her post. This combination can make for somewhat "fun" bickering sessions, and even change (hopefully for the better) how we perceive what is being discussed... because you are essentially looking at things in a similar way, but you are interpreting them differently/approaching them differently, which is how I see squares playing out often in my own (short) experience with synastry.
But, with the opposition, because they may have their Mercury just opposite my sun, that's another ballgame entirely. These are often people who can emotionally hurt me with their words/phrasing... probably due to the fact their Mercury is then opposite of mine but sextile my Moon at the same time.... Plus, it is Cancer to Capricorn, and I already have the aspect of their Mercury and possibly Sun's ruler in hard aspect to my Sun's ruler... so the emotional influence they have with their words is deeply magnified. (I have this aspect with my Baba, I do think it has something to do with why I always took what she said to me, as her "hating" me, when later when I finally spoke up about it (years later), she confessed she's never hated me, but loves me unconditionally, like any grandma should, right! lol But, how she says things, and how I hear them... are so different, that we ended up having a lot of strained relations and long absences because of the ill-understanding... (Mercury opposite Sun* both ways)
The way the quincunx seems to me to play against communication, I can say has been with annoyance... lol that person usually says things which to me, are interpreted as being too light-hearted/without consideration, or too random (idk if it is felt this way by them too though)... and then I feel confused by them... what is interesting is that for this aspect to happen, it means their Mercury is either cnj to my MC or to my Chiron.... XD admittedly, my friends with their Mercuries here... they do have a habit of turning into the Charlie Brown-style adults that go "WhaWHA wha Whah wawaWha" instead of using actual words... it's annoying, and I do have a tendency to unintentionally "zone out" when talking with them. lol
A semisquare to my Mercury can be the same as either the quincunx or a full square effect... maybe they're effected by transits more... since their not "wholly" squares... what I mean is... I've noticed with people who have Mercury semisquare to my own, that at certain times we can communicate just fine... but at others we just don't agree on anything, and stumble on our words... I think somehow it ties into transit Mercury, or other forces at play... it's hard to tell! lol
Squares to the sun are for me, the worst... these people don't usually understand that they're saying something that rubs me the wrong way... IME that is... astrology helps though - and has~ cause once you know that's what is where the tension is coming from, you know how to avoid it, the "stepping on toes" or "talking over one another" and the senseless arguments... lol I think what helps, in my particular case... is that when someone has Mercury square to my Sun, at least they have the sextile/trine from my Mercury in our favour! (my Mercury is very early in Capricorn, my Sun is very late in Capricorn) --- it all depends on the orbs, eh?! ^^;
In case that their Mercury should square my Moon, I can be a bit sensitive to that person... it's not a super big deal, IME... not at all like the ones who have Mercury square my Sun... but, there is definitely something there which leads to communication errors... misreading the other persons intentions, and ideas...
I don't often feel or think, I misunderstand others, at least not too often. But, there are definitely times, and situations where it has happened,(often inopportune)... lol Maybe, it's having Mercury as the chart ruler, and dominant...
But, on the other hand... I'm always a bit lost with how I come off sounding, or how I word things and how they receive them/will receive them...
- that's Mercury semisquare Saturn though... plus, my Mercurial communicating, can be kinda erratic... my humour is drier than death valley, and I know how easily what I say/how I phrase things can and does sound narcissistic, pompous... shallow... sometimes... >.<; Cappy-Merc >.<;
Anyway - that's how I see those moments of, "dude.... wth are yo' talkin' `bout?" Oo;?