The tons of undefined behaviour in C that ought to have been implementation defined instead, and increasingly sadistic compiler writers on the other side.

Like signed integer overflow that should just do what the underlying machine does, i.e. in practice, wrap around two's complement.

But the wierdest UB is when a C source code line has a non-matching ' or ". WTF, this should have been a compile time error!

The crazy shenanigans you can do with C++ standard libs are fascinating.
Like implementig multithreading with just a foreach, and bindings which can make member function pointers to simple function pointers, and placeholders in bindings. Also lambda functions are cool.
Something between the lines:
my_crazy_class *tmp = new my_crazy_class(...);
std::vector<type> my_array = .....;
std::for_each(std::execution::par,my_array.begin(),my_array.end(),
[&](type in){
auto fn = std::bind( &my_crazy_class::my_crazy_fnc,*tmp,_1,random_static_value);
return fn(in);
});
ps:
It's pretty much pseudocode, and please don't do things like this, it's bad for your mental health.
pps:
I need to learn how to use this tools wisely.