A colloquial expression meaning 'definitely' or 'no doubt'; equivalent to 'certain' or 'sure'.
あのホテルは人気だから、満室で間違いなしだ。
That hotel is popular, so it's sure to be fully booked.
彼の説明を聞けば合格は間違いなしだと思う。
After hearing his explanation, I think passing is certain.
この方法なら失敗は間違いなし。（※皮肉や軽い強調で使うこともある）
With this method, failure is impossible. (Note: can be used ironically or for emphasis.)