Only On yom Kippur

Only on Yom Kippur are you allowed

Shulchan Aruch (O. Ch. 63: 2 )   Someone, who is sitting
and is מחמיר to stand up for קריאת שמע,
is called an “עברין” (sinner)
Mishna V’rurah s.k. 8  writes, "The Elyah Rabah in the name
of the Maharshal, “On Yom Kippur everyone* agrees, you
are allowed to stand up. (so to imitate the מלאכים)”

*The Pri Megodim disagrees

 I forgot there are two more things that 
may be done only on Yom Kipper

1) According to the Magen Avraham  & Mishna Verurah. (565) 

There is only one day in the year (Yom Kippur) that 

the Piyut האדרת והאמונה,לחי עולמים may be said בציבור 

In Nusach Sfard (Ari)  it is said every Shabbos. 

They don't follow this Psak of the  Magen Avraham &/M.V.

Nusach Ashkenaz says it only on Yom Kippur

2) Saying י"ג מדות & סליחות in the night time before חצות

is only allowed once a year, on Yom Kippur.