OS: Linux Mint 18.3 Sylvia
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.17.13515 (Git)
Build type: None
Python version: 2.7.12
Qt version: 4.8.7
Coin version: 4.0.0a
OCC version: 7.2.0
Locale: English/UnitedStates (en_US)
This may be specific to this particular distribution of Linux.
This has been around for several versions of FreeCAD, and several versions of Linux Mint Cinnamon. I was surprised when I searched and found nothing, but perhaps I chose the wrong search terms.
How to reproduce:
Open a sketch
enter any constraint that takes a length or angle (requires text entry, and allows equations)
Click on the equation button (or hit '='), note that the original value does not go away.
Type in your equation, note that you have a double exposure of the text you just typed in, and what was originally there.
This does not prevent proper operation, it just makes it really hard to read the equation you entered. I find I need to highlight it to make it readable.