In the Parshah of Chayei Sara, in Bereishis 25:6, there is a Posuk that refers to הפילגשים - "the concubines". Rashi expounds that since the word is missing a Yud, it must be a reference to the fact ...
Some names are popularly used in a different form than the way they're spelled or pronounced in Tanach. A few that I can think of are: Nochum (נָחוּם) - is written in Tanach (Nah. 1:1) as נַחוּם ...
Why and by whom was there a yud added to make "ירושלים" instead of the original "ירושלם"?