If the array function is entered in multiple cells (instead of a single
cell), you need to select all of the cells before editing the function. As
already stated, after editing you must commit w/Cntrl+Shift+Enter
Because 9 times out of 10, most folks will make a revision, and then
*automatically* hit <Enter>, which with this type of formula, will invoke
the error message "cannot change part of an array".