The set-up: place both files in a directory, add ZeroClipboard 1.2.3 and Require.JS so that they're found.
Open testcase.html and watch console output: Sometimes the text is copied, sometimes not:
22:16:08.959 "Copied text to clipboard: " testcase.js:5
22:16:10.141 "Copied text to clipboard: a" testcase.js:5
22:16:11.544 "Copied text to clipboard: a" testcase.js:5
22:16:12.057 "Copied text to clipboard: " testcase.js:5
22:16:12.816 "Copied text to clipboard: a" testcase.js:5