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A JavaScript file with the text  a<b/c=''  will cause unexpected behaviour.

1. It prevents the enter and tab keys from working anywhere in the file.
2. You can not remove the first character with the backspace key.
3. Adding a open bracket ( in between the  a and the < will hide the last character in the document which was viable. The ( itself will not show but seems to be there. Removing it using the backspace key will make the last characters in the document visible again.

 It is possible for this text to exist spread over multiple lines of code.
For example
a<b

//this is a comment

c=''

will also cause the unexpected behaviour as well.