Coitus, Coital
Even the usually punctilious Sheldon Cooper and his Big Bang chums can be heard to pronounce it 'coy-tus', whereas if you visit the Oxford Dictionary Online website and click on the speech icon, a male British voice will state that it is 'co-it-us'. Likewise, the pronunciation of the word 'co-it-al'/'coy-tul'
There is a third variation, although less popular I think: 'coition'. Now by applying the same precept, 'co-ish-ion' makes sense, whereas 'coy-shun' just sounds wrong!
Unlike 'aluminium' and 'aluminum', which are technically both correct in one sense, although different in another, and 'Tom-ay-to' and 'Tom-ah-to' depending on which side of the Atlantic Ocean that you happen to be on; 'po-tay-to' and 'po-tar-to' have only ever existed in the mind of George and Ira Gershwin.
Another example which appears to be in quite common usage, is the word 'buoy' which Brits and I believe Aussies pronounce 'boy', whereas Americans say 'boo-ee'. Since this is a nautical term and shares a root with 'buoyant' and 'buoyancy', I will let you make your own mind up which is correct.