this is an old issue from bugs.plos.org, apologies in advance if this is a dupe.
in firefox, if you try to annotate the author contributions in the acknowledgements section (which i'm assuming is a forbidden area) you get taken to the top of the page where a javascript alert is waiting for you.
the text of the alert is the same text you see if you click the 'add annotation' link without selecting anything:
"using your mouse, select the area of the article you wish to annotate"
the error should explain that the chosen area is not avaiable for annotation
in IE you don't even get an alert. however, the little 'javascript error' icon appears and if you look at the console you see your old favorite IE javascript error:
Line: 628
Char: 5
Error: 'id' is null or not an object
