And not really a mnemonic - stationery with an e in it for Envelopes. Two shops near here have stationAry printed on their shop fronts. I suspect they used the same signwriter.
Necessary - Never eat cake eat salmon salmon ary
Difficulty - Mrs D, Mrs I, Mrs F F I, Mrs C, Mrs U, Mrs L T Y.
These were 2 from my school days and have never forgotten them.
Another one was the main causes of unconsciousness:
F Fainting
I Infantile convulsions
S Shock
H Head Injury
S Stroke
H Heart Attack
A Asphyxia
P Poisoning
E Epilepsy
D Diabetes
Colour coding bands on electrical resistors - remembered from school days - there are perfectly useful neurons that have been idling around for a lot of years waiting for someone to ask this ...
It's not spelling but if there's a question about camels and how many humps they have, a Dromedary has one and a Bactrian has two. If you look at D and B, 'D' has one hump and 'B' has two.