Description:
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Unsetting an element of an array unsets that element in copies of the array. This does not happen if you change the array or its copy in some other way after making the copy and before doing the unset. I haven't tested whether this affects other non-atomic types like objects, but it wouldn't surprise me if it does.

My educated guess is there is some copy-on-write optomization that (wrongly) does not consider "unset" to be a write.

Reproduce code:
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$x = array(1,2);
$y = $x;
# uncommenting the next line "fixes" the bug:
#$x[0] = $x[0];
unset($x[0]);
print_r($y);

Expected result:
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Array(0=>1,1=>2)

Actual result:
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Array(1=>2)