Here are some of the myths that I've seen.
Myth: You must replace your cup every year/every five years/every X years.
Reality: You probably don't.
Cups that are made from silicone could last indefinitely. Cups that are made of latex will start degrading before cups that are made of silicone, but they still have a long shelf life. If you've passed whatever magic point your particular cup company says and your cup is still working well for you, I wouldn't worry.
Myth: You must use our branded wash/tablets to clean your cup.
Reality: There are many ways to safely clean your cup.
While there is nothing intrinsically wrong with using a product like Diva Wash or Lunette Washing Liquid or what have you, they work about as well as any mild soap or "intimate wash" would. There are also other ways to clean your cup. Some people use diluted vinegar, isopropyl alcohol or hydrogen peroxide. (Please note that there are some concerns about using bases and acids (such as hydrogen peroxide and vinegar) to clean silicone. However, the occasional soak in hydrogen peroxide probably won't make your cup wither away to dust.</a>) You can boil your cup in water - I've seen companies quote anywhere from 2 to 20 minutes for this. And some of the brave among us just use water!
Myth: Your cup must sit at the bottom of your vagina, way below your cervix.
Reality: Your vagina may vary.
Since a cup isn't inserted at the same angle or to the same place as a tampon, most of the cup companies use diagrams like this to show where the cup should be placed:
(brazenly stolen from the Diva Cup website)
In fact, some cervices are right at the entrance of the vagina when their owners are menstruating. In addition, some people's cups "ride up" so that they're right around the cervix even if the cervix is higher up. As long as it's not leaking and you're not experiencing any discomfort, there's no problem if your cup and your cervix are right next to each other.
Myth: There are only one or two ways you can fold your cup when you are inserting it.
Reality: There are at least eight different folds you can use. And probably more, if you use your imagination!
For some reason (probably due to lack of space in the pamphlets and lack of knowledge), the cup companies only ever show one or two folding methods (usualy the C fold and the punchdown fold). While these folds work for many (for example, this poll seems to suggest that the C fold and the punchdown fold are the most commonly used folds), they are wrong, wrong, wrong for others. I suggest using the folding methods tag to find one that works for you if you can't get the cup in or get it to open up properly.
Myth: You have to rotate your cup to make it seal correctly.
Reality: Rotating your cup is only one way to make sure it's sealed.
A couple of the cup companies (and Diva is notorious for this) say that in order to get a proper seal, you have to rotate your cup. While this is an excellent way to ensure you have a seal, it's not the only way. Kegel exercises or swirling your finger around the outside of your cup are both ways to ensure that you have a good seal. Again, if it's not leaking and you're not uncomfortable, you're fine.
Myth: You can't use any lubricant with your cup.
Reality: Water-based lubes are fine, but they must be water based.
What the crap? I was just browsing through the Diva Cup instructions and found this one. And I quote: "Some lubricant ingredients can be damaging to the silicone, so we suggest using only water as a lubricant! Otherwise, the silicone may be compromised and may ruin the cup." Funny, because LadyCup actually SENT lube with my girlfriend's cup!
Water-based lubes are safe for use with cups, but make sure that the lube you are using only contains water-based lubricants. Silicone-based lubricants will degrade silicone, and oil based lubricants probably will. There are also some concerns about glycerin in lubricants as a yeast infection/thrush risk, although your mileage may vary on that one.
And, of course, there's nothing wrong with using water as a lubricant if that's what you're comfortable with!
Myth: The size guidelines are hard and fast.
Reality: Not really.
As was mentioned above about cup position, vaginas do vary. Some women do not have as toned pelvic floor musicles as others. And some women have super-toned muscles, even after vaginal childbirth. In addition, the guidelines vary wildly between companies. (Diva, for example, says that women should buy the larger cup if they are over 30 or have been pregnant. But LadyCup suggests that women over 25 should use their larger size.) There are also other factors to take into account, such as the height of your cervix and the heaviness of your flow.
(As a side note, while researching for this I discovered that Lunette appears to have revised their sizing guidelines to reflect what I'm talking about here. If any Lunette staffers are reading this - kudos!)
Myth: Menstrual cups can only be used when you're on your period.
Reality: As long as you remove and clean every 12 hours or so, you'll probably be fine.
In countries where medical approval (through government bodies like the FDA or TGA) is required, menstrual cups are usually submitted and approved as menstrual collection devices only. As a result, this is the only use that they can endorse. However, there are folks (including some in this community) who use their cups "off-label" to catch discharge during other parts of the month. While we are not medical doctors (well, except for elizabby) and we can't really endorse the safety of doing this, but we can say that people do it and without any obvious ill effects.
This is by far not an exhaustive list, so if you can think of any other myths, please mention them in the comments!