Apparently not commas. Depending on how you have WordPress configured (see below), using a comma, period or perhaps some other punctuation marks in your post name will break the link to the post. It seems to be just good practice not to use a comma or period in your post title and to avoid any unnecessary marks as well including colons, quotation marks and slashes.
To see how you have WordPress set up to create the links to your posts (and pages):
1. Go to the Dashboard and click on Settings.
2. Under Settings click on Permalinks.
3. If you have Permalinks set to include the name of your post, either change it to some other format, or be cautious when creating post titles and avoid unnecessary punctuation.
Two more suggestion:
1. Make each post name unique. Having all your posts named “Room 25 Newsletter” doesn’t say anything about the content of the post and it doesn’t help user find posts when searching.
2. Make post names short but expressive. Using action verbs in the name makes it more interesting. A post name of “Science Projects” might be improved with “Science Projects Dazzle Parents”. The name for this post might have been “Commas Can Break Your Links”.