Ignore the fact that having more keywords in C++ precludes the legality of some C code being C++. (`int class;`)
void * implicit casting in C just works, but in C++ it must be an explicit cast (which is kind of funny considering all the confusing implicit behavior in C++).
C++20 does have C11's designated initialization now, which helps in some cases, but that was a pain for a long time.
enums and conversion between integers is very strict in C++.
`char * message = "Hello"` is valid C but not C++ (since you cannot mutate the pointed to string, it must be `const` in C++)
C99 introduced variadic macros that didn't become standard C++ until 2011.
C doesn't allow for empty structs. You can do it in C++, but sizeof(EmptyStruct) is 1. And if C lets you get away with it in some compilers, I'll bet it's 0.
Anyway, all of these things and likely more can ruin your party if you think you're going to compile C code with a C++ compiler.
Also don't forget if you want code to be C callable in C++ you have to use `extern "C"` wrappers.