This one came up from a client who had lost some activities.
Turns out she had created a Company record with no Contacts then scheduled a to-do for that record.
User's expectation, the To-Do would be associated with that Company.
Actual result: the activity is scheduled with whatever contact detail record was last opened.
Suggestion: when scheduling from any entity other than a Contact record there should be a warning pop-up "Do you really want to schedule with this contact?" to alert the user that they need to confirm who the activity is scheduled with, not just OK the suggestion offered!