slip up=mistake

[ostap77]

"I knew I was going to slip up on this question. It was a most difficult one."

If I made a mistake, I would slip up on something? Does it always take the preposition "up"? Can I say slip on something?

[bhaisahab]

"I knew I was going to slip up on this question. It was a most difficult one."

If I made a mistake, I would slip up on something? Does it always take the preposition "up"? Can I say slip on something?

You could say that you made a slip (noun), or that you slipped up (phrasal verb).

[ostap77]

You could say that you made a slip (noun), or that you slipped up (phrasal verb).

So it's slip up on something or make a slip on something?

[ostap77]

So it's slip up on something or make a slip on something?

Any comments?

[JMurray]

So it's slip up on something or make a slip on something?

ostap.
You can "slip up", "slip up on something", "slip up somewhere" or simply "make a slip". They are mostly interchangeable.
Examples:
"I knew I would slip up on something, and it turned out to be the algebra question".
"I knew I would slip up somewhere, and sure enough I didn't leave time for the last page".
"I knew I would slip up/make a slip, and sure enough I got question two wrong".
And of course one can "make a slip of the tongue", a mistake of some sort (often revealing or embarrassing) while speaking.

not a teacher