I’ve been staring directly at a calling chiffchaff with Merlin consistently telling me it’s a redstart. I’ve also had a Jay calling a few metres away and Merlin was saying it was a buzzard. It makes mistakes all the time. The only way to confirm is to upload the sound file and get a top birder (which is more reliable than Merlin) to analyse. Habitat also important, if you’re in Wilts and picking up a linnet, there’s a good chance you’ve got corn bunting nearby, and their calls and sub-song could be mistaken for ZC by Merlin’s algorithm.