Mobo died. Got new one with fancy new dual core processor and latest swanky DDR2 RAM to go in it. Came home and found that my older power supply unit didnt support the motherboard, the connection to the motherboard had four pins less than required.
Went back to the shop (15 minute drive), bought new power supply. Came home and casually looked at old power supply to find a separate four pin connector that could be tagged along to the rest...
Quietly went back and returned the shiny new unit. Nice folks that they are, they took it back. It helps to have had bought stuff worth > 100,000 Rupees from them till then, and since then I have matched that amount and then some.
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On a softer note, While trying to limit the range of variable x to ±10, I coded:
If x < -10, x = -10
If x > 10, x = 103.
This led to almost disastrous results and if it hadn't been for the quick thinking of a colleague (in reverting to manual mode) I'd have set my project back by months... and I was only trying to change limits from ±13!