I'm assuming you're on android (since I've never heard of this elsewhere) - was this with 2.5.0 or 2.5.1?
If you still have 2.5.0, check for the update - there was at least one known cause of this in that version. If this happened with 2.5.1, if you figure out any way to reliably make it happen, please post the steps to the bugs and tech forum.
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