I've built up an old Kirk frame as an 'interesting' commuter for winter to keep my steel single speed nice. Commute is only 3 miles so performance isn't high on the agenda.
Took it round the garden and I thought I'd left something loose, checked it over and couldn't find anything out of place. It felt like the front wasn't connected, I found that I could move the seat tube laterally just by holding the bars and leaning on it. How did this ever make it to market? It must have been fascinating being a Bike journo back when stuff like this was coming out.
Check out this clip of the frame flex.
I haven't cut the grass between the slabs as it's at the wildlife end of the garden and it's full of froglets and newtlets before the commentators chip in...