Do you ever feel invisible? Well, Democratic presidential candidate Sen. Bernie Sanders might feel your pain.

During an event in St. Louis, former Secretary of State Hillary Clinton asked where Bernie Sanders was in the '90s when she was fighting for health care reform.

Things got awkward when a member of Sanders' campaign tweeted a screenshot of the Vermont senator literally standing right behind Clinton as she spoke at Dartmouth College about health care reform in 1993.

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Clinton spokeswoman retorted, "Exactly, he was standing behind her. She was out in front."

The Sanders campaign has used snapshots from Clinton's past against her before.

Back in January, Sanders tweeted a thank-you card from 1993 from the then-first lady. He potentially tweeted this as a jab at Clinton for attacking his single-payer health care plan.

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Not every tweet is contentious, though. The two also occasionally use Twitter to support each other. Clinton notably retweeted Sanders when he went after Republican front-runner Donald Trump, saying, "America's first black president cannot and will not be succeeded by a hatemonger who refuses to condemn the KKK."

And Sanders retweeted Clinton when she sent out a link to donate to Flint, Michigan, which is mired in a water crisis.

This video includes images from Getty Images and clips from C-SPAN and NBC.