McDonalds Dude: Can I take your order?
Jeff: Can I have a hot fudge sundae please?
McDonalds Dude: No, the machine is broken.
Jeff: Awwwwwwwwwwwwwwwwwww!!!
Heartless Friend: Hahahaa, what a McLetdown.
(noun) an event that may cause one party a source of entertainment due to the frustration of another party specifically-wanting something (food or toy) from McDonalds.
E.g. Keito was pissed off because they no longer served hashbrowns when she arrived at McDonalds about half a second too late! This McLetdown only provided amusement to her sibling who drove her there.
Looking forward to a McDonalds icecream all day only to find when you get there that the icecream machine isn't working/is empty/is being cleaned (although THAT one doesnt happen much) so you miss out.