Here's a list of mildly interesting things about the C language that I learned over time. There are many more mildly interesting features of C++, but the language is literally known for being weird, whereas C is usually considered smaller and simpler, so this is (almost) only about C.
1. Combined type and variable/field declaration, inside a struct scope [https://godbolt.org/g/Rh94Go]
struct foo {
struct bar {
int x;
} baz;
};