What are common problems with web servers and strategies to counter them?

I'm learning internet technologies and my lecturer has been asking this question for a while. I have read everything I can find on web servers - what are they, types of web servers such as Apache, Tomcat, have even created my own but I just can't get an answer to this question.

It's not an exam or a test, I'm just curious. If any of you can shed some light on the matter, I'll be really grateful.