Partway through a loop, the program stopped because it exceeded a soft limit

I've been working on this subroutine to surface my router table spoilboard (attached). It's a loop that starts at the current origin and works its way up and down Y, stepping over along X by a fraction of the tool width.

The Y moves are relative. The X moves are absolute and controlled by the loop variable. It worked once without issue. I ran it again, and it stopped after a few passes saying the move would exceed (or exceeded?) the machine's negative Y soft limit. How is this possible? 

No help, but I am equally puzzled.

Which version of LinuxCNC are you running?

There was a problem with G53 and rounding errors at one point, but your G53 isn't in the loop.

My guess is there is an offset on the Y axis from a previous program or the program that called the subroutine.