- The Trace
When J.K. (as in “Just Kidding, Harry doesn't really die) Rowling introduced the concept of the Trace in Harry Potter and the Chamber of Secrets, it was clear that she didn't really understand the rules for her own magical universe. (“How about, if Harry uses magic, all the car alarms in a 50 mile radius go off?”)
When Dobby uses magic to levitate a pudding in the Dursley household, Harry gets a warning that threatens expulsion the next time he crosses the line. Fine. We can ignore for the moment that magic is less precise than a GPS unit or the kind of tracking device you put in your pet.
If you hate them.
The problem isn't so much that the magic is vague and imprecise, the problem is that the magic is inconsistent.
Let's fast-forward to the fifth book (Harry Potter, the Angsty Little Bitch) where Harry has been expelled (this time for actually doing magic.) He is warned by several adult authority figures not to do any further magic under any circumstances.
Except, you know, special occasions