I saw this in a television advertisement two days ago.

Problem:
“Someday” is an adjective, but an adverb is required here.

Explanation:
The verbiage displayed in the FedEx TV ad was along the lines of “Chance that someday you will forget to hit mute”, with no space between “some” and “day”.

“Someday” — without the space — tells you what.

For example, a “someday occurrence” is something that occurs at an indefinite future time.

In other words, “someday” is an adjective; it modifies a noun (such as “occurrence”).

In contrast, “some day” tells you when.

An example of the correct use of “some day” is “It will happen some day.”

The goal of the television advertising copywriter was to tell the viewer when he or she would forget.

Solution:
“Chance that some day you will forget …”