How about this…
double GetTheNumber()
{
// get the important number from somewhere
}
And then elsewhere…
var theNumber = GetTheNumber();
DoSomethingImportant(theNumber / 5);
And then, at some point in the future, somebody notices that GetTheNumber only ever returns whole numbers so refactors it to return int rather than double.
Bang! No compiler errors and you start seeing unexpected results, because what was previously floating-point arithmetic has now become integer arithmetic without anybody noticing.
Having said that, this sort of thing should be caught by your unit tests etc, but it’s still a potential gotcha.