Here is my attempt to demonstrate that $1=0$ .

1) $0=0$

2) $0=1-1$

3) $n\cdot 0=n(1-1)$ , $n$ is a big natural number.

4) $0=\underset { n\text{ times} }{ \underbrace { (1-1)+(1-1)+...+(1-1) } }$

5) $0=1-1+1-\cdots -1$

6) $0=1-(1-1+1-1+\cdots)$

7) $0=1-((1-1)+(1-1)+\cdots +(1-1))$

8) $0=1-(0+0+\cdots + 0)$

9) $0=1$

In which step is the first error committed?

The answer is 7.

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From step 5 we observe that the last term is $-1$ . However in step 7 if we expand out, we realise that the last term is now $1$ , not $-1$ . Hence first mistake in step 7