I have a couple of young students and ALL of them have used this one:
"But I can play it perfectly at home! Promise!"
Like we can't tell if they have been practicing or not... *sigh*
Oh yeah. Today I had an interesting case like this:
Student (12 y old) with
desperate facial expression and a lot of sighs: "uhmm you know...this piece...I think it's just a bit hard...I mean, I don't get the harmonies. They're weird.

" (Etc.etc...)
I listened a bit and noticed that she had perhaps practiced 3 times in two weeks, as she was slowly deciphering the notes, AGAIN ignoring all of those fingerings I had suggested, crescendo-wise, during the three last lessons!!
Do I really need to explain why I got slightly angry? 
Anyway, shortly afterwards: "Oh my, you know this...this...well it just doesn't come to me!!"
Me: "Yes of course it doesn't just come to you, you have to do something for it, you have to read properly, and don't ignore the fingerings that I wrote for you, and first of all, you might even want start to
think a bit and don't expect to be spoonfed like a baby and don't behave like a 4year old, as you are 12!!" (and btw she is actually very intelligent, otherwise I wouldn't have gotten angry in the first place)
Student, a bit later: "Well, when I played it during practice it just came to me easily, and now it doesn't"
Me:

Well now, first you tell me it doesn't come to you, now you tell me it did come easily to you, so what is now the truth? It can't be both, as you are evidently contradicting yourself!"
....
(Student
finally stops acting like a drama queen and starts to work on that piece)