Countless times throughout my life. In fact, a big part of my life is slowly deprogramming from years of propaganda. Whether it is religion or politics the amount of misinformation is enormous as it is prolific.
Even something very personal like relationships is fraught with tons of negative cultural issues around control and love. Most of what society teaches is a lie designed to perpetuate things like the Patriarchy.
Edit: After reading a lot of these I would like to offer an alternative to what a lot of people have said.
I learned about a conspiracy back in the early days of computing that was essentially that the US was intercepting all emails and all phone calls around the world.
There was a lot of good evidence including a spy pact with Canada where we had an installation on their soil and they had an installation on ours so we could spy on our own citizens without breaking the letter of the law.
Also good evidence that AT&T and other providers had let the government access their major server trunks to install their own hardware.
Well Snowden proved it was all real. This was probably the biggest conspiracy theory of my lifetime.