Having used virtualization in a variety of scenarios over the last 10-15 years, I still find some misconceptions about the value proposition and how to use it. At best these are just marketing misconceptions, but at worst they can lead to counterproductive activities that HURT your solution. So, not in any order, here three things I hear people say that are just normally not true. Yes I understand that there are some scenarios where they are, but for the most part in my experience these ideas have been taken too far and now create problems.
Virtualization helps me scale my solution
I hear this so much I really just get tired of reexplaining how this isn't true. While based on a grain of truth, in the general sense at the datacenter level, it's completely false. For a single application it is easier to add cores to a virtual machine than it is to buy new hardware...but...people forget that the hypervisor is running on real honest to goodness hardware and adding a new Virtual Machine on existing hardware actually REDUCES the capacity of the other virtual machines. Couple this with over provisioning and you could end up where 4 virtual cores giving you the capacity of a single core (or less). Worse yet, if you're just the "customer" you may need 4 virtual machines to get the capacity of a much smaller piece of hardware.
Hardware virtualization doesn't have any overhead
This is just silly, of course it does... Sure hardware is going to be much lower overhead than software, but scheduling Virtual machines on and off of shared CPU, Memory, Cache is overhead...again if over provisioning against underlying hardware is your "enterprise scaling strategy" be prepared for performance impact. Virtualization has overhead.
vMotions are undetectable and have no performance cost
I normally don't use profanity, but there's one word for this "Bullshit" (OK, maybe that's two words). For some reason, this lie propagates to the point (I blame vmware marketing for doing too good a job) that people have an almost religious belief in the "all holy magic" of Vmware;s ability to magically move state of a virtual machine with "zero impact on performance". Just stop believing, while it's transferring and maintaining state of a virtual machine from one piece of physical hardware to another...things slow down. End of story, don't believe me, drive a virtual machine to some nontrivial level (and measure the application performance...i.e. how LONG do my non trivial business transactions take?...don't forget that...many people do) and the motion it...if you're lucky it will only be a minute or two of "holy crap! what happened?"