It seems that any activity created in 4.7.1 (not sure about earlier 4.7 versions) sometimes shows all formatting tags when viewed.
I reproduced this both in Wordpress and Drupal demos by creating a new case. When viewing the "Open case" activity, the < p> tag was shown, as were tags for strike-through, italics and bold, as well as h2 heading. Same happened when adding and then viewing a single activity added to a contact.
What is strange is that adding or editing an activity to an existing case went fine, no tags were shown in the subsequent view.
As I was able to reproduce this on the demo, I believe it is a bug in CiviCRM. It can be rather big issue, since it can render activities with more formatting nearly impossible to read. I'd report this as a bug to jira directly, but instructions say first post here, so here I post.
edit: The affected activities also show all tags on their print versions (of the single activity).
edit2: I would even try to somehow fix this myself with my limited php/html knowledge, but I can't even find the file responsible for this. :(
edit3: Just a small update, issue is still present in 4.7.2, possible related/same issue on Jira