Elon Musk chimes in after Microsoft and Nvidia gain as much as $192 billion in market value in a day. His reaction could be a red flag to investors.

• Microsoft and Nvidia gained as much as $192 billion in market value between them on Tuesday.
• Elon Musk reacted to the pair's massive increase in market cap by tweeting: "Crazy times."
• Musk once tweeted that Tesla stock was overvalued, and complained about its volatility.

Elon Musk underlined the astounding scale of Microsoft and Nvidia's combined market-value surge on Tuesday with a two-word tweet: "Crazy times."

While Musk didn't spell out whether he views Microsoft or Nvidia as overvalued, his strong reaction could be a red flag for investors.