Is it permitted to teach Torah is gentiles are present? Sources can be found here. 

Is it permitted to perform mitzvod in a bathroom? What about repaying a loan in the bathroom using Zelle? Sources can be found here. 

If one forgot יעלה ויבא on Rosh Chodesh but doesn't realize until after he davened mussaf, does he repeat shacharis? If he does, should he put his tefillin on again? Sources can be found here. 

Is it permitted to cut a cake with words on it? Is it permitted to open a book with words written on the sde of the pages? Is it permitted to assemble a puzzle on Shabbos? Sources can be found here.