‘The plugin generated 272 characters of unexpected output during activation’

[Rumperuu]

When activating the Plugin (3.0 branch) on my test site, the following message is displayed:

The plugin generated 272 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.

[jcvignoli]

Hard to guess without the actual test plugin, but my take:
1/ you left 272 empty characters after a php closure (?>) in a php file that is called upon activation.
2/ there is an error/confimation message displayed on screen in one of the files called upon activation. It's always hard to debug, but try to check the development console in firefox/chrome, you might find a clue.