I should 'of'...

I'm in the middle of marking final-year exam papers for students completing a BA in Languages. Many of these students will go on to become teachers.

I've been doing this for long enough to have got used to the typical malapropisms and spelling slips on words like "affect/effect", etc. and a fairly general ignorance of the use of apostrophes and other punctuation. But this year, for the first time, I came across students systematically writing things like "dessision" and "nessisary"...

Im like, OMG, like...
 
Peter O'Mahony at the weekend kept talking about how Munster had "left" their fans down. I'm a huge fan of his but why on earth do people say "left down" instead of "let down"? It makes no sense.
 
Maybe he means the fans were 'left down in the dumps', 'left down in the slough of despond' ?

Or just 'left the competition' ?
 
Maybe he means the fans were 'left down in the dumps', 'left down in the slough of despond' ?

Or just 'left the competition' ?
That's nasty
I would have loved to see Munster and Ulster make it through to the knock-out stages, especially this year given that it's the first year of the new competition which was imposed by the English and French Club owners who wanted a bigger slice of the cake despite the damage they have done to European Rugby.