1. This is not related to recent changes regarding skip-pen-up-down, because it also occurs in v. 2.7.4 of eggbot.py. While attempting to debug the issue, I downloaded v. 2.7.5 so further debugging was/is with this version.
2. May well be timing related, but it IS reproducible ( perfectly, without fail in several dozens of trials) here...at the 2950th node count of layer 2 of the attached .svg file. Accessed thru the Layers tab of Eggbot Control.
3. A debugging statement inserted just before the python error issues at the last line in this snippet
   shows that strButton is in fact of zero length, so Python is correct in its error statement: "IndexError: string index out of range"

    strButton = ebb_motion.QueryPRGButton(self.serialPort)  #Query if button pressed
    if len(strButton) == 0:
        inkex.errormsg( 'NULL strButton' )

    if strButton[0] == '1': #button pressed [ERROR HERE WHEN strButton IS NULL]

cottage06 before stretch after anti-bulge+stretch.zip