Under the right circumstances, people use "under" instead of "in"

Arnold Zwicky over at Language Log has been looking into the phenomenon of "under" versus "in" occurring before the phrase "[modifier] circumstances". I, personally, couldn't recall ever hearing someone say "in the circumstances", but Zwicky stated previously that not only is this how he says it, but it's considered proper.

Of course, it's Language Log's purpose to debunk prescriptive language rules, so he did a little googling to see how people are actually using the phrase. Not content to simply check with "the circumstances", he tried "these", "all", "no", and several other modifiers.

In summary: the Google data suggest that "under" is preferred to "in"

(modestly)
    with determiners "the" and "these"
(more strongly)
    with determiner "which"
(very strongly)
    with determiner "what"
(almost categorically)
    with quantity determiner "no"

but that "in" is preferred to "under"

(almost categorically)
    when "circumstances" means 'personal situation'
(strongly)
    with determiner "those" in general
(almost categorically)
    with determiner "those" plus certain following relatives
(modestly)
    with quantity determiners "all" and "some"
(strongly)
    with quantity determiner "many"
(almost categorically)
    with quantity determiner "a few"

This just scratches the surface of the phenomenon, but it's enough to indicate that several effects are probably going on. As usual, the facts of usage are complex, subtle, sometimes surprising, and not easy to derive from first principles.

Ah, science.