Here is the issue… (yay, lesson :D )
-The TI-OS has a parser to read TI-BASIC code.
-This parser has two main modes: Program Mode and Equation Mode.
-Program Mode is for reading programs. Some commands don't work in Program Mode but most do.
-Equation mode has many more restrictions on allowed commands and is basically used to evaluate simple expressions (Think: Y=X^2 kind of stuff). This mode also allows the OS to parse non-programs (like strings and equations)
-When you use expr(), two things happen. The calculator enters Equation Mode and then it basically hacks the parser to make it parse a string
-Now, when you have an error in Program Mode, you can go to the code, using the editor. When you have an error in Equation Mode, if it is a Y= equation, you go into the editor to the error. However, there is no editor for strings, so it just goes to the homescreen. Annoying, right? I am pretty sure an asm program could fix this (until a RAM clear), but maybe an OS patch could permanently fix this.