Yes.

Methadone is an opiate so if you add opiates on top of opiates you are more likely to overdose.

Methadone is not an opiate antagonist but it does tend to block the euphoric effect (intense pleasurable feeling) of other opiates when it is at an appropriate dose. This can be dangerous if people use even more opiates to try and get the effect they are looking for. 

You also increase the chance of overdosing while you’re taking methadone if you mix it with sedatives like benzodiazepines or alcohol.