Bug introduced in 10.4.1 or earlier and persisting through 11.1.0 or later
Defer generates strange results.
Evaluating the following commands one by one (in three different input cells), with Shift+Enter, one gets:
x (* 15 but I have also seen 17! *)
Even stranger, if one creates a notebook with only those same commands, restarts the kernel, and does
Evaluation ► Evaluate Notebook then the command
1 instead of