Why do wonders abruptly stop?

[ernesto]

about 50% of the time when I start building a wonder, it stops, saying that I can't complete them. Worst yet, they usually stop when they are about done (<5 turns to go).
This happens throughout the ages, no matter if I have a ton or money or no money.
My question is, if I can't complete them, why are they offered?
What am I doing wrong?
Also, how do I go about conducting the especial espionage missions (I know only how to steal technology and research city).

[Todd Hawks]

Some other Civ was faster than you and built the wonder before you. And as each wonder can only be built once....

For the special espionage missions you need to have researched espionage, built the intelligence agency and planted a spy (Shift-Ctrl-E).

[ixnay]

If you press F7, you can see who is building what wonder and where...