well... for me...
Corsa - spending more than twice the value of the car on it and it still being slower than a milk float, however stopped and handled very well and i still miss it to this day just wish it was quicker.
Ibiza - well... buying it was a big mistake as its the worst car in the world handling wise, however it has redeemed itself since moving to a job thats a 50mile round trip each day as it averages 58 to the tank which is nice however still doesnt make up for the god awful handling, also bought alloys for it which weighed more than the moon and sold them for a massive loss after buying new tyres for them, also bought rear anti roll bar + spacers for it, spacers were too wide so wheels scrubbed, anti roll bar was good and improved handling however car had a tendancy to face the roundabout whilst going around it :| so they all got sold and now its back to its standard form.
zed - depends which way i look at it, mistake was buying it as its an expensive toy now and money really should have been put towards the future however everytime i drive it puts the biggest smile on my face ever so in that respect its not a mistake.