1.   Buying that house was a big mistake.

2.   Buying this car was a big mistake.

3.   His big mistake was to brag to one of the fat traders how he had done it.

4.   I had made a big mistake.

5.   I had made a pretty big mistake in how I handled it.

6.   I shall argue that this reasonable conclusion is actually a big mistake, and that loss of vision makes things even worse.

7.   I told her she was making a big mistake but she was too pig-headed to listen.

8.   Part of her had the feeling that she was making a very big mistake.

9.   Ralph is making a big mistake.

10.   Setting them up with one another proved a big mistake.