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 1469A - Regular Bracket Sequence multiple candidates misunderstood the requirement of exactly one '(' and one ')', perhaps because the brace symbol itself was not quoted. Without quotes () looked like a part of sentence to some.
The following examples could be used for such problem to improve clarity
Too many braces
Not enough braces
Input overly long
1337 ..... ..... .....