There are some situations when Medicare is not primary. This usually happens if you're currently employed and have coverage under your company where they are primary. In these cases it is important to watch for your EOB (Explanation of Benefits) from your insurance companies, because many medical billers are so used to billing Medicare first, that they will automatically do that and Medicare will deny the claim because of it and if you do not catch it in time, you could end up paying the bill the deadline for filing in a timely manner has lapsed.
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If you only have Medicare, or Medicare plus a Medicare supplement plan, Medicare is primary and your Medigap plan is secondary.
If you are also covered under an employer group health insurance plan, and your employer has fewer than 20 full time employees, Medicare is primary.