Question: [Sunday 20th Tamuz, 5780]
I accidentally wore my Rabbeinu Tam tefillin for Davening instead of my Rashi’s and only noticed when I went to take them off and put on my “Rabbeinu Tam”s after Davening. What should I do?
In such a case simply wear your Rashi Tefillin after Davening and say the paragraphs that you would usually say with your Rabbeinu Tams. You should say a blessing on your Rashis in such a case, although it is bets to try to use the bathroom beforehand.
Sources: See here for the full details of this subject: