Harriette says to Laura 'Do you have any idea how hard it was to get you to 16?', but Laura turned 15 three seasons earlier.
Church bells do not work the way shown in the bell tower scene. They are used to indicate the hour (a.m. or p.m.,) and thus do not chime more than twelve times, which it does several more times than twelve. Thus Urkel and Carl had no reason to panic about not being able to get off it, as it should have stopped moving after twelve chimes. Also, there is one instance where the bell moves once but gives two chimes.
It's hypocritical for Carl and Harriette to be overprotective with Laura for not allowing her to stay overnight with her friends they know to the concert while they're confident with Eddie to allow him to go to Canada for 4 days with people they don't know. Laura is usually responsible enough to earn their trust while Eddie is usually gullible and incompetent.
Harriette says that she still thinks like her parents but her father walked out on the family when she was 3 and Rachel was 1.