HCl is titrated with NaOH. When doing the titration, some of the NaOH splashed onto the inside surface of the Erlenmyer flask, and you forgot to rinse it into your sample. Would the systematic error be falsely high, low, or unaffected and why? Thanks.

As I understand the problem, you have HCl in the Erlenmeyer flask and you are titrating with NaOH from a buret. So if you don't wash all of the NaOH into the HCl, you will be adding more NaOH from the buret to make up for that which stayed on the walls of the flask. That makes the NaOH volume too high. How would that affect the molarity of the HCl?
vol HCl x M HCl = vol NaOH x M NaOH.
M HCl = (vol NaOH x M NaOH)/vol HCl