When commands like /summon or /effect are used to summon an entity or give an effect, typing in a special character before the entity type and after the colon, or before "minecraft:" returns "Unknown entity: minecraft: " instead of showing the entire text.

Another behavior is that if any special character is before the colon in "minecraft:", the output is "Unknown effect: minecraft:minecraft". This behavior is also shown if you replace the colon by the character.

For example, typing in /summon %asdf returns "Unknown entity: minecraft: ".

Typing in /effect give @p minecraft^speed returns "Unknown effect: minecraft:minecraft"

Something else I noticed while testing is that only the colon in "minecraft:" is required and you don't need to type out the entire "minecraft:" when using commands like /summon, /effect, or /give. For example, /give @p :stone works. Not sure if intended, but doesn't appear to be a big deal.

The only characters I've noticed that actually get returned along with the entity or effect name in the error message are: 

-

_

/

.

Link to video: https://youtu.be/4xHsLGtFFGE