If I recall correctly I had six different Spanish teachers over all my time studying the language in the United States, and they all lied to me. Every. Single. One. A few examples of the atrocities imposed on my Spanish:
I learned that this is called a pluma,
A car is not a carro (that would just be way too simple), it is a coche. You don't manejar a car, you conducir. What I learned is a computadora is called an ordenador in Spain. Angry is enfadado rather than enojado. Don't even try telling a Spaniard that a tortilla is this:
In retrospect, these are silly little vocabulary differences which reflect the differences between Castellano (Spanish Spanish) and Mexican or Latin American Spanish (Quite parallel to what I'm experiencing with English differences now). I grew up and studied Spanish in Southern California, which logically means most of the Spanish I learned was Mexican. I have to give some of my teachers credit for teaching both pluma and bolígrafo, among other examples, but it wasn't until coming to Spain that I thought about why I learned two words that mean the same thing. Here, one of them is the proper term they use, and the other is hillbilly language.
These little confusions are fixable. I retain one of the words and store the other one away for future use in a more appropriate location. I can forgive my teachers for this much. What I cannot forgive is providing me with incomplete grammar that affects the fluidity of my language.
By far the worst lie I was told: "No one uses the vosotros (second person plural) verb form and you're never going to need it, so we're going to pretend it doesn't exist." At the time I thought that seemed great. I only have to learn how to conjugate verbs in five ways rather than six. Sweet! Boy is that coming back to haunt me.
My Spanish grammar is, thanks to my education, horribly incomplete for use in Spain (You know, that place where the language came from). Spain definitely uses vosotros. I often get to really awkward points in conversations when my words are flowing, I'm carrying on nicely using preterite, future, and even subjunctive verbs. I might even sound like I can speak the language. But then, bam! Out of no where I reach a point where I want to use the second person plural, and I have to stop and think about how to do so, even in the most basic present tense. Teachers use vosotros all the time in addressing groups of students or the class as a whole, and I'm painfully conscious of my words when ever I do so. I've taught myself all the different forms of vosotros, but it will still be a while before they are ingrained into my head the way the rest of the verbs are. The chant in my head is still "soy, eres, es, somos, son" when it should really be "soy, eres, es somos, sois, son."
A plea to Spanish teachers all around the world: please teach your students vosotros. They just might find themselves in Spain some day.