Bahrain's man-made "Passport Island" remains unclear as to whether the border runs down its middle or to either side - I even wrote to the Bahrain government to ask them where their border was and they couldn't tell me either! Can anyone confirm one way or another? (If it does go down the middle, quite a few quizzes on this site will need rewriting). I've always treated the India-Sri Lanka "border" as non-existent; it seems to depend on whether the tide is in or out, and whether a sandbank has been built up or washed away in the latest monsoon season.
The Johor-Singapore Causeway is man-made, rather like Passport Island on Bahrain and/or Saudi Arabia. I don't know if it is a true causeway or has a bridge component. I'll add an "excludes man-made causeways and islands" to the instructions.
What about Cyprus with Akrotiri and Dhekelia? My reading is that they are British territory (as opposed to Guantanamo Bay, which is technically leased Cuban territory).