I had that happen once on my 88 300E, but as tkamiya said in his post I shut the car off and restarted it to clear the condition. It has never come back. I am currently having several climate control problems that I believe are related to bad solder joints in the CCM (climate control module).
I think that the flashing recirculate LED may also be related to this condition. I intend to remove it and resolder it soon in hopes that it will fix all of the gremlins. these gremlins are: no heat at the footwell vents (until I hit a bump) slow response to changes in the temp setting. AC only, after the car sits in the sun and the interior is hotter than outside to get heat I need to move the temp wheel to full hot then back to a normal temp. All of these symptoms seem to be due to poor connection between the switches, temp control wheel and the CCM.

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Dave
88 300E 250K Sold
2000 C230K Totaled @104K
2003 ML320 156K Sold
2009 E350 4Matic Sport 46K Sold
2011 ML350 60K Sold
2014 GLK350 46k Sold
2012 GL450 67K
2016 E550 Cabriolet 48K