From @zoffixznet

It seems .flatmap method does not flatten.

Running the example in the docs produces wrong result​:
<ZoffixWin> m​: say ((1, 2), <a b>).flatmap(&uc).join('|');
<camelia> rakudo-moar a563e8​: OUTPUT«1 2|A B␤»

Also, this​:
<LLamaRider> m​: say ([1,2],[3,4]).flatmap({$^x}).perl; say ([1,2],[3,4]).flat.flatmap({$^x}).perl;
<camelia> rakudo-moar a563e8​: OUTPUT«($[1, 2], $[3, 4]).Seq␤(1, 2, 3, 4).Seq␤»

Relevant IRC conversation​: http://irclog.perlgeek.de/perl6/2015-12-26#i_11781049

From @zoffixznet

More cases with flat too​:

<AlexDaniel> m​: my @​a = (1, 2), 3; say flat @​a
<camelia> rakudo-moar 9441bb​: OUTPUT«((1 2) 3)␤»
<AlexDaniel> m​: my @​a = (1, 2), 3; say |@​a
<camelia> rakudo-moar 9441bb​: OUTPUT«(1 2)3␤»
<Zoffix> | is slip
<AlexDaniel> m​: my @​a = (1, 2), 3; say flat flat @​a
<camelia> rakudo-moar 9441bb​: OUTPUT«((1 2) 3)␤»
<AlexDaniel> m​: my @​a = (1, 2), 3; say flat flat flat flat @​a # dammit
<camelia> rakudo-moar 9441bb​: OUTPUT«((1 2) 3)␤»

http://irclog.perlgeek.de/perl6/2015-12-27#i_11787064

From @niner

Arrays itemize their elements. Itemized are never flattened, even when explicitly flattening the array. You need to recursively walk through the nested data structure either by converting everything into Slips or using gather/take or something else.

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