This case is a little bit more complicated. Let's have a look at the second person singular and the first person plural.
written and spoken
"regular" but wrong
"regular ending "
Because some personal endings starts with i or consists only of an i (i, iamo, ino) we would have two following i, one which softens the c and the other one from the personal ending. These two i were melted to one i the throughout history. It is a philosophical question whether this is a real irregularity or not. In any case the second i is neither pronounced nor spelled nowadays.