One time I wanted to go up to Lechmere and back just to see Medford test trains terminating there, so I got off a northbound D train, waited around a bit, and got a southbound E without exiting the platform. Technically this could be considered fare evading, because I got off at a station without gates and got on in the other direction, and there were T employees around, and none of them cared, despite me clearly not coming from/going to the mezzanine to get a receipt.
Back when the Union Sq branch first opened (don't call it just the "Union Branch" like that T video does...), I got a receipt at Union, rode to Lechmere, got off, looked around, including exiting the station mezzanine, so when I re-entered, the ambassadors told me to get a receipt before heading up (like they did to anyone entering either stop the first week or so of service), but upon showing my receipt from Union Sq, they said it was fine, the receipt was valid for 90 minutes, and let me go straight up. This was the only sort of enforcement I've ever seen on the GLX, and those ambassadors definitely aren't there anymore to make sure every goes to a fare machine before heading to the platform.