Bug introduced in 8.0 and persisting through 10.3.0
It appears that in debug mode, the
\[Piecewise] symbol and its associated grid give a syntax error. For a minimal reproduction, enable the debugger, and type in the front end:
Esc pw Esc
Ctrl-Enter (or Ctrl-Return)
Note that the grid must be typed using the keyboard. Inserting it using the menu does not trigger the bug.
Is it a bug? Are there ways to fix that except write it in the input form?