If you're counting on Medicare to cover your healthcare costs after you turn 65, you may be in for a nasty financial surprise that could derail your retirement.

Even for seniors who purchase optional Medicare supplementary plans, healthcare costs could be as high as $350,000 for a couple during their retirement years. And, that scary number doesn't even factor in long-term care costs and is based on the current benefits system. Costs could be much greater if Congress changes Medicare. 

Why does Medicare leave seniors on the hook for so much healthcare spending? Here are three big reasons you can't count on Medicare to keep you out of the poorhouse if you have costly health issues after age 65.