Left front tire without any warning went flat, in changing it the shock or strut had broken and punchured the inside wall of the tire. Is this something that is known to happen or is this a rare occurance?

if its bouncy /saggy or feel all bumps more than likely a shock you can look a the shocks if their is oil showing on shock seals went and you have a blown shock also bent or broken bolts or connection will cause same

Automobiles: 2006-2007 Ford Five Hundred 2WD & AWD, Montego

If the steering is getting a little hard to turn at low speeds, this would indicate a loss of Power Steering, and the steering is reverting to "Non-Power" steering, just like the days of the Ford Model T or Model A. Loss of power steering is usually caused by low level of PSF (Power Steering Fluid).
So first on my checklist would be checking the PSF level in the reservoir.
Also bleed (air) from PS (Power Steering) pump.

The Ford Five Hundred has front MacPherson struts, so check these for leakage (of the dampening fluid in the strut cylinder), or breakage of their coil springs.
These springs can actually weaken over time, but this is impossible to see - you must "feel" it by bouncing the strut up & down - preferably with your hands on the top of the strut tower in the engine compartment (rather than on the exterior finish).
If the MacPherson struts have weak/broken springs, these components are not separately serviceable, and the strut would have to be replaced as a unit (and preferably both left/right to maintain smooth/even ride).

The Five Hundred also has rear shock absorbers with springs - check for leaks/breakage/weakness here too.

Steering losing responsiveness/wander - checklist
Priority Action Part Type Cause
1 Inspect Rack & Pinion - Center Wear From Internal Fluid Leaking Around The Rack Piston.
2 Inspect Steering Stabilizer Bracket - Loose or Broken Steering Stabilizer Bracket.
3 Inspect Front Lower Control Arm - Front Lower Control Arm loose/missing bolt(s)
4 Inspect Front Lower Ball Joints - Front Lower Ball Joint(s) Worn/Loose
5 Inspect Front Upper Ball Joints -Front Upper Ball Joint(s) Worn/Loose
6 Inspect Tire Pressure - Tire Pressure should be 35 PSI for the Five Hundred and Montego's P225/55R18 OEM (Original Equipment Manufacturer) tires.

Harsh ride - checklist

1 Inspect Rear Shock/Strut - Shocks/Strut failing/leaking.
2 Inspect Front Shock/Strut - Shocks/Strut failing/leaking.
3 Inspect Coil Spring - Incorrect Coil Spring Selection or Installation.
4 Inspect Coil Spring - Coil Spring broken.

The strut tower is basicly a shock with suspension spring built in.Look behind your front tire and you will see a big spring with shock inside.That is the strut.

My guess would be bad ball joints. Raise the front of the car and place it on safety stands. Find a long board or some type of tool that you can put under the front tire and pry up on the tire. If the ball joint is bad, the tire will move up and down. You could also get some movement with a bad shock/mcpherson strut.

HONDA Pilot struts do not require replace at specific mileage intervals or maintenance. Your vehicle’s struts should be checked annually. Struts wear out slowly and you may not notice gradual losses in ride comfort, handling and control. Excessive bouncing, rocking back and forth, drifting or nose- diving while braking, swaying, or cupping wear on the tires are some signs that your vehicle may have worn Struts. If you are purchasing new Struts for your HONDA Pilot consider premium Struts. Upgrading your struts usually improves overall ride quality, control and handling. Warn struts can increase the wear of tires, ball joints, steering linkage, springs and C.V. joints. Some original struts may be getting weak after 40,000 or 50,000 miles.

Failure symptoms: Vehicle continues to bounce after hitting bumps in roadway. Front suspension dives severely during hard braking. Swaying or rolling when at higher speeds. Signs of oil leaking from the shock rod seal. Clunking and banging noises coming from the strut may be signs of internal shock failure or strut mounts.

it has struts in front and shocks in rear have a professional do a front end alinement and check strut and shock perfomence they may have to road test.

I would suspect you have a broken or worn out shock or strut. If it is bouncing in the rear or front or one is broken replace both shocks/struts. They must be replaced in pairs. You may have to replace all four, depending on their condition. If you see a leak they definitely are worn out. Worn shocks/struts will cause your tires to cup and wear out prematurely. They will also cause your vehicle to handle poorly especially in an emergency maneuver. This can be very dangerous so do not neglect worn or broken suspension parts.

the strut only need replacing if there is alot of bounce in the car, or if there leaking, or the main reason is there broken. that would be the only reasons to replace struts

jack car up and see if the tire moves around easy.

it can be many thing shock,top plate ,stabilisator link , good luck pierre