One of C’s not so brilliant features is the range of Undefined Behaviour (UB). Things like using an uninitialised variable or having an int variable overflow don’t have behaviour defined; thus it is UB and you cannot accurately predict what will happen. Likewise accessing a NULL pointer can cause odd behaviour. It gets more sophisticated than that. What if you type cast an int to a float?
The LLVM blog have an interesting set of posts on UB and it’s definitely worth reading. I’ve done a lot of C programming, so I’m rarely surprised by my programs but its useful to know about these things. In my case, I started with assembler programming and then learnt C++ and C in that order so my perspective has always been to try and understand whats going on deep down.