I had a fishtailing issue for over a year now, and it got to a dangerous point where my car would fishtail in the rain on turns after hitting almost any road imperfection, even at low speeds. Wasn't very stable in the dry either, even in a straight line, but at least the car didn't want to put me in a ditch. Went crazy with a prybar earlier this year, looking over my suspension for damaged bushings and spherical bearings, anything bent, etc. Finally got the idea to tighten things down, but I just did the usual suspension stuff. Found some of the nuts on the rear trailing arms could use 1/4 turn tightening, and some control arm bolts needed some snugging up. But that still didn't solve the case.

Finally decided on tightening up EVERYTHING I could see that was between the body and the wheels, and found the rear crossmember bolts to be quite loose. Some by over a full turn! That solved 95% of the issue, and the rest is probably just slop in old bushings. My car feels reliable again after over a year of wondering and nail biting in the rain!

Even if you don't have an issue as severe as mine, if you haven't tightened anything up in the past few years, good idea to give everything a rundown. Might make your car feel more solid at worst.

 

Great point. With my older (20+) year cars I tend to tie this search for loosness with a car cleaning. Roughly once a month or so I wash the cars top to bottom. As I get to each section of the car, I tend to start slow by looking at the fasterners, bolts, etc in the area to see if anything might seem off, missing, loose, and sometimes I randomly put a wrench on to see how this are. This has saved me countless times with little stuff like the inner fenderwell linings missing thos plastic fasteners that just seem to fall off randomly, tires with a buuble or nail stuck in them, loose exhaust, etc. it helps that I keep a box of fasteners in the garage so I can immediately remedy the lost connector. I'd guess about every 3-6 months I also lift the car to clean underneath and also check all the bolts, examine bushing and look for leaks underneath.

Good work identifying the problem, even a shop would have a hard time finding that. I only two days found a loose ground wire, with no idea how it could have backed out the bolt and I was surprised there I had no errors yet with the car. Reconnected and on my way.

 

Yeop, it's called 'nut and bolt'. Do it before track events, during track events and on typical putt-putt machines that have high mileage.
It's a newer problem that has only recently been kind of addressed, since modern vehicles with EFI started to last longer (200-300K miles) this problem crept upon the public.
But most folks can't tell when their lug nuts are loose, wheel bearings are shot, or if they put the gas cap back on or closed their fuel door.

Anytime you are under the car, it is always a good idea to inspect and nut/bolt the car while under there.
GC/GD Imprezas are notorious for front end slop from loose bolts, but you can't tell that to the kiddies over at NASIOC.