There are various arguments, some dating back to the 1960s, which show that quantum field theories cannot allow any fundamental particle to exist with spin>2. But "string theory" has arbitrarily high spin "particles." Contradiction? I had asked several physicists about this over many years & none provided a good answer. However, allegedly the answer is this. The inconsistency arguments assume (sometimes implicitly) that the number of particle/field types in the quantum field theory is finite. String theory escapes by having infinite set. Essentially, each time there would be an inconsistency, it is canceled out by putting in an interaction with even-higher spin particles which cancel out the inconsistency. Due to infiniteness, the chain of such fudge-corrections need never end. Still, I think this represents a very important/frightening way in which string theory is fundamentally different from quantum field theories, deserving of note. This all is not mentioned in most (all?) of the popular books "explaining" string theory to the unwashed.