GPS fail / spike

Hi all,

on my yesterday's run I had a negative experience with GPS. 
It was a structured training from my training plan at training peaks.
First 1:05 h easy running, then 4 repeats with strides followed by 6 minutes of cool down.

Just a few seconds before the end of fourth break - watch started to beep already - I stopped recording because I reached bakery.
After one minute of shopping, I pressed resume I got warning that I'm too quick - speed someting around 1:20 min/km.
But I was continuing at about 5:30 km/min pace until the starting point.

Back home the track of the activity looked like this - having a big spike to a place a couple of km away:

The weird thing is that I saw somthing similar at a friends track on Strava.
Has anyone else experienced such behaviour?