Model and code co-changing is about the coordinated modification of models and code during evolution. Intermittent inconsistencies are a common occurrence during co-changing. A partial co-change is the period in which the developer changed, say, the model but has not yet propagated the change to the code. Inconsistency feedback can be provided to developers for helping them to complete partial co-changes. However, there is no evidence whether such inconsistency feedback is useful to developers. To investigate this problem, we conducted a controlled experiment with 36 subjects who were required to complete ten partially completed change tasks between models and code of two non-trivial systems. The tasks were of different levels of complexity depending on how many model diagrams they affected. All subjects had to work on all change tasks but sometimes with and sometimes without inconsistency feedback. We then measured differences between task effort and correctness. We found that when subjects were given inconsistency feedback during tasks, they were 268% more likely to complete the co-change correctly compared to when they were not given inconsistency feedback. We also found that when subjects were not given inconsistency feedback, they nearly always failed in completing co-change tasks with high complexity where the partially completed changes were spread across different diagrams in the model. These findings suggest that inconsistency feedback (i.e. detection and repair) should form an integral part of co-changing, regardless of whether the code or the model changes first. Furthermore, these findings suggest that merely having access to changes (as with the given partially completed changes) is insufficient for effective co-changing.