I am receiving the following error when trying to run this update query:

Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

UPDATE tbl_I_Engagements
SET IsMaster = 0,
MasterID = 1604
WHERE MasterID = 87426

It is a simple update with NO subquery and I don't understand why I'm getting the error. There is multiple records with MasterID = 87426 but I should be able to update multiple records at a time! I feel stupid posting this because it is probably some easy fix but I've been looking at it for too long!

Maybe you should pick a platform appropriate forum..

What platform is it?

An d just as an shot from the hip, I'd say MasterId is not unique

I figured it out! It was a trigger that was attached to the table.