Strange print anomoly

I was printing this, when at the beginning of the wall section the printer abruptly shifted the X&Y position about 1.5mm. Then, as it printed approx 10 more layers it gradually moved back to the correct position. The model did not move, the bed did not move, the belts are tight on the XY platform, power is supplied from a UPS, Slic3r shows the correct view without any jogs. I am using Repetier Host running from a laptop with a good battery. Any idea what I might look for? I just can't picture anything that would cause that immediate shift then a gradual return to the correct co-ords.

happened to me too. i had to scrap the print, but tried again and it worked. weird stuff

what kind of printer do you have. Mine is a coreXY