I think the title provides enough advice.
I just worked on a ticket where Sitecore 8.1 Experience Editor was breaking in some pages. Breaking as in the components/placeholders borders and backgrounds were not visible. So you get the feeling the EE has not worked at all, although you can click on fields and edit them. Our FE dev actually reported the EE is not working at all.
After investigating the cause, we were able to narrow down the cause to a single template field which by chance was lately updated by me to add Help long/short description that included a pair of double quotes (“text”).
The moment I removed the double quotes our EE worked normally. I tried switching to single quotes instead of double and it also worked.
I also submitted this issue to Sitecore Support.