A previously posted solution, taken from my Web page without permission, was removed by the admins at my request. The person who posted it should have asked first and should have credited me. Here is my solution:
This message has several possible causes.
- If you get this when making some kind of plot, you probably
forgot that you have one of Plot1 Plot2 Plot3 turned on,
and it refers to two lists that don't have the same length. The cure
is to press [Y=] and turn off the unwanted plot.
- If you’re actually doing something that needs lists, such as
a regression, but your lists don’t have the same length, you get
this message. Press [STAT]  and check
- You also get this message when you give the TI-83 a list or
matrix where it expected a variable or number, or vice versa.
For instance, on the STAT PLOT screen, you may mean to press
[2nd] [L1] but if you miss the
[2nd] you actually type a Y. In the same way you might
type a Z where you intend to type L2.
- Karl Wein offers another possibility. You can
create a list called L1, but that is not the same as the
predefined list called L1 (note the
small-capital L), and of course similarly for lists 2 through 6. If
you accidentally delete a predefined list from the editor, make sure
to bring it back with the SetUpEditor
In general, when you see this message you need to check
through what you’ve done to make sure that you used lists where
you were supposed to, and nowhere else.
I am copying this from my own Web page for my students at