You want to know something that makes me mad? Those three words: 40 lines max. That's the seemingly standard maximum number of lines in a poem -- lines between stanzas included!-- that poetry editors want to see in submissions.
God forbid you should write a good poem with 41+ lines!
It's not every editor, but recently I came across three such restrictions in a row. I will name and shame them:
(1) The Bridport prize: "Maximum of 42 lines"
(2) Mslexia's "New Writing" feature: "strictly up to 40 lines each"
(3) The National Poetry Competition: "Poems must not exceed 40 lines (not including title)"
The National Poetry Competition is particularly egregious in applying this restriction because they didn't use to (check out the archive of past winners on their website). So why now? And it's ironic too because "Poetry Review", the quarterly journal of the Poetry Society, applies no such limitation.
I'm tempted to find some others but it will only irritate me more.