Avoid if Possible

Using void * because this generally means that one must using casting, and in C++ casting is rather ugly. It is OK to use void * to pass structure address where the structure is not known to the routines accepting the packet (typically callback routines). However, declaring void *buf is a bad idea. Please use the correct types whenever possible.
Using undefined storage specifications such as (short, int, long, long long, size_t …). The problem with all these is that the number of bytes they allocate depends on the compiler and the system. Instead use Bacula’s types (int8_t, uint8_t, int32_t, uint32_t, int64_t, and uint64_t). This guarantees that the variables are given exactly the size you want. Please try at all possible to avoid using size_t ssize_t and the such. They are very system dependent. However, some system routines may need them, so their use is often unavoidable.
Returning a malloc’ed buffer from a subroutine – someone will forget to release it.
Heap allocation (malloc) unless needed – it is expensive. Use POOL_MEM instead.
Templates – they can create portability problems.
Fancy or tricky C or C++ code, unless you give a good explanation of why you used it.
Too much inheritance – it can complicate the code, and make reading it difficult (unless you are in love with colons)