On the android app, the delete icon (trashcan) is right next to the complete icon (checkmark). If you have it set to auto-hide completed tasks, it is very easy to delete a task instead of complete it. And if you aren’t watching closely enough to notice the strike-through just before it goes away (or lack thereof), you don’t realize that you just deleted the task.
My preference would be to have a configuration setting to enable or disabled a confirmation dialog (ie, “are you really sure you never want to see this task again?”). At the very least, move one or the other icon so that they aren’t sitting side-by-side!