In the "Pater Noster" prayer, why is "panem nostrum cotidianum" sometimes written as "panem nostrum quotidianum"?
Quotidianum and cotidianum are simply two different ways to spell the same word. This word belongs to a group of words that can be spelled with either a QU or a C, which includes cum and quom.
These interchangible spellings are probably due to a similar sound (in at least one dialect of Latin) between QUO and CO or QUO and CU. The same thing is happening a bit further down in the prayer with the spellings caelis and coelis, because the pronunciation of AE and OE sounds the same in some dialects.