Mishnah Berurah — משנה ברורה

It's as if an interruption was made - From the fact that he (the Mechaber) was not specific, it's inferred that he's referring to even [a case] where all the four cornered garments were placed in front of him at the time he made the blessing... and even there he would have to go back and bless again... for it proves that he usually only wears one (since he didn't have them all in mind)... and therefore without specifying, his blessing does not count for them all... for it is like he just decided [on a whim] (to wear more than one)... unless he had them in mind explicitly. And the same rule applies if he made the blessing of "על מצוַת" on his טלית קטן, as is our practice, and immediately after took (wore) his טלית גדול... if he had in mind explicitly to exempt it with his "על מצוַת" [blessing], then he does not need to go back and bless; But [if he made the blessing] in general (ie. without having in mind explicitly), he goes back and blesses again... for [when done] generally he only has the טלית קטן in mind when he makes the "על מצוַת" blessing. Nevertheless, ideally one should be careful not to make a blessing on the טלית קטן if he plans to wear the טלית גדול right after, for he is causing an unnecessary blessing [to be made]. If one wore a טלית without checking it and found out [after] that it was invalid, or that the ציצית strings became invalid afterwards... and he wears another טלית... he needs to bless again... unless he had in mind explicitly at the time he made the blessing that it should cover everything else that he [might] wear afterwards.