Sun square Mercury transit is such a busy time mentally that so much interaction with others, and so many ideas popping into your head, can make it challenging to make sense of everything.

The information overload can lead to scrambled thinking, so you may have trouble expressing yourself clearly. This can lead to arguments with other people because you are not making sense to them or saying something you do not mean.

These other people in your life may also unsettle you if they question your plans or thinking on a particular subject. Listening is better than talking right now, and this is not the best time to make crucial decisions, even though you may feel pressured. This is not the best time to sign contracts or make business deals.

This stirres up many Accusations and Criminations against the Native, as counterfeiting some Writings, or guilty of such abusive Actions; denying falsly what may be legally proved, whereupon much Infamy fals upon the Natives discredit either by not paying Moneys, or by forgery, or counterfeiting or clipping Moneys; the Mind extreamely afflicted, and losse of Office, if it be in bawdy Courts, or Spirituall; an aversenesse to Study; oppressed with Clamours, and variety of unjust Asperions.
      How the Sun can come to the opposition of Mercury, I confesse my selfe ignorant, unlesse men live almost one hundred yeers or more. [Christian Astrology, William Lilly, 1647, p.685-686.]

This interpretation for Sun square Mercury transit can also be read for a Solar Eclipse and New Moon square Mercury.