For quite some time I am using codeforces for candidate screening in job interviews.

One common suggestion to improve problem statement clarity is to add examples of invalid inputs to problems with constrains.

For example in this problem: 1469A - Regular Bracket Sequence
The following examples could be used

Too many braces

  1
  (())

Not enough braces

  1
  (????????????????

Input overly long

  1337
  .....
  .....
  .....