What you think it means: A horrible event, a tragedy
What it actually means: A distorted representation of something
This may be one of the most misused words on this list. Horrible wars or mass killings aren't necessarily travesties. A travesty is more like a parody or mockery of something; it's not a tragedy. For example, if a murderer avoids jail time due to a technicality, you could call it a "travesty of justice." A movie parodying a famous genre can be considered a travesty, even if it's lighthearted. As for your prom night? That remains a travesty. Some things never change.
What you think it means: Being bored with something
What it actually means: Indifferent, having no feelings
Being disinterested in your guinea pig doesn't mean you are bored with it, it means you have no feelings toward it at all. In other words, you give exactly zero fucks. This is a problem, because guinea pigs need regular feedings and water. Use "uninterested" instead and keep that lil' guy alive.