At file scope:

int this_is_a_fairly_long_variable_name_1;

int this_is_a_fairly_long_variable_name_2;

Why? Because section 5.2.4.1 requires implementations to support at least 32 significant characters in external identifiers and section 6.4.2.1 says "If two identifiers differ only in nonsignificant characters, the behavior is undefined."

These ones may be more "obvious", but:

Given e.g. int a[4][5] then accessing a[1][7] is undefined even though one might assume it's like accessing a[2][2]

x << -3 is undefined behaviour though logically one might assume it should be equivalent to x >> 3

And the classic, very common newbie use of "fflush(stdin);" is also undefined.