if(you.type == NULL && entity1 && entity2 && my.state == 1)

causes this error:
can not translate P_ANDAND:LONG:POINTER:LONG

this way it works:
if(you.type == NULL && (long)entity1 && (long)entity2 && my.state == 1)

can this be fixed?

Yes. '&&' is a boolean operation that requires boolean expressions. A boolean expression is something that's either zero or nonzero. "entity" is a pointer and not a boolean expression.

Correct me if I'm wrong, but a pointer == 0 is true and pointer != 0 is false in C, so this is valid syntax.

btw the compiler errors are kinda cryptic, this could be improved.

i also thought in c you can use pointers in if clauses like that and i agree that lite-c error messages could be better.

why would most programmers recommend against it? what problems can it cause?

if(entity1) // but if you only use a single pointer and no && it is ok?

I probably overstated myself. Most C programmers are probably perfectly okay with it. But C programmers are crazy and enjoy hunting down memory leaks and memorizing which operator has a higher precedence.

IMHO, sane programmers try to avoid this.

In the example above, you are assuming that the compiler will automatically cast your pointer into a boolean value. Automatic casting doesn't always work the way you think it will.

This isn't an issue with most C compilers because it is a common practice to cast pointers to int, but it is still a cast.