(embarrassing) blunder / mistake, embarrassment
[noun]
[de fla-ter, de fla-ters]

A “flater” is a stupid mistake in front of other people causing embarrassment. It’s more or less synonymous to “blunder”, which also translates to blunder, but the subtle difference is that a “blunder” is not necessarily witnessed by other people (causing the embarrassment).

A “flater” is mostly used in the expression “een flater slaan”: (lit.: to hit a blunder) to be embarrassed / to blunder.

Examples:
– “Frank liet een scheet tijdens de vergadering, wat een flater!” 
(“Frank farted during the meeting, what a blunder!”)

– “Ik sla echt een flater met dit bodybuilderschort…” 
(“This bodybuilder apron really looks embarrassing on me…”)

– “Met de publicatie van dit onverkwikkelijke artikel sloeg de krant echt een flater.” 
(“The newspaper really blundered by publishing this distasteful article.”)

Expressions:
–
“Voor lul staan/lopen”: (lit.: to stand for cock) to look like an ass.

Example:
– “Je rits staat open…” – “Oei, dan loop ik al de hele ochtend voor lul.”
(“You’re fly is open…” – “Ouch, I’ve been looking like an ass all morning then.”)

Related words:
– Gênant: embarrassing [adjective].
– Lullig: embarrassingly stupid [adjective].

Example:
– “Frank ziet er een beetje lullig uit met die koffievlek op zijn hemd…”
(“Frank looks a bit stupid with that coffee stain on his shirt…”)

– Prutser: incompetent blunderer, screw up [noun] [de prutser, de prutsers].

Extra:
“Guust Flater” (in the picture) is the Dutch translation of Gaston Lagaffe, a comic strip created in 1957 by the Belgian cartoonist André Franquin. Check this article on Wikipedia for more info.