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 72K For Sale
2016 E550 Cabriolet 48K Sold
2015 GLK350 114K