That's correct, you can insult someone accidentally while complimenting them in a similar way. The particles は (as in wa) and が (ga) have different connotations that can simply different things.

So saying メリーさんの顔はきれい (Mary-san no kao wa kirei, "Mary has a beautiful face") causes an implication that Mary has a beautiful face, (... But nothing else about her is beautiful). Changing the は for が makes the statement come across as intended.

Without going into detail on the whole wa vs ga thing, wa is more like "as for x..." which can imply a "but..." at the end, whether stated or not, which causes this effect.