How could this happen? I planted an entire bed of flowers from a bag of seeds labeled ‘mixed annuals’. (Seeds were bought from a local chain/hardware store. ) They grew like weeds! I thought I had finally planted something my wife would like.
Not so much.
Only one kind of plant came up. Grew as dense as anything I’ve ever seen. Grew about 3 1/2 feet tall and had a green tuft on the top. Never flowered.
A farmer neighbor told me it was RAGWEED. Wife is sneezing her head off.
Has anybody else ever had this happen?
Did you water them? 
I’m guessing that the ragweed seeds were already there in the garden bed and overpowered the seeds you planted. That’s how weeds end up classified as weeds - by growing more vigorously than the desirable plants.
One way to suppress weeds in a garden bed is to use the ‘lasagna method’ where you layer newspaper or cardboard in the bed and then put fresh topsoil, compost, leaves, or other organic material over it which you plant the plants you want into. That way, any weed seeds in the soil will be smothered by the newspaper and the desired plants will have a chance to grow before weeds get a foothold in the new soil. More info here if you want to try prepping a bed that way now for spring planting: http://forums.gardenweb.com/forums/load/newgard/msg0900160519930.html
lavenderviolet beat me to it; has some additionally good gardening advice, too.