Costs for serving a s21 incorrectly?

Possession - Section 8 and Section 21

In a recent post on the Forum, ‘Dash’ [11th Jan 13] mentioned a case where he had a wrong date on a s21 so his case was rejected.

He writes that he received a general form of Judgement or Order letter from the county court stating he must pay the defendants cost (state funded solicitors)!

I have heard of landlords who filled out the s21 incorrectly having their cases thrown out of court.
I have never heard of them having to pay for defendants cost.

Is this something that you have heard of before?
If not then is this a mistake or could it theoretically be a way that some courts are now trying to make money? Are you aware of any legislation regarding this?

If this can be charged, is there a reason why some [most] courts don't ask us to pay these costs, and why some now do ask us to pay these costs? Is there anything we can do to ensure that if we did make a silly mistake we will not be lumbered with even more costs?

Thanks
ZTA