There's grass in the lettuce bed!
Wherever possible, I plant cover crops when a section of the garden is done for the year. These are non-harvested crops that are grown for their beneficial effects on the soil - depending on the variety they can capture excess nutrients, add organic matter, prevent erosion, block weed establishment, fix atmospheric nitrogen in the soil or even loosen compacted subsoil. But in some cases, beds will open up too late for a cover crop to establish itself before the cold sets in. For example, this bed of lettuce won't be finished until mid-October, several weeks after I would need to sow oats (a good fall cover), which really need to be in by the third week of September to make a meaningful impact.
The solution is to interseed the cover crop with the main crop. Since the beds are (generally) weed free, this just means seeding between the rows a few weeks before harvest. The main crop is usually fairly tolerant of competition when it's close to harvest, so it's really just a net benefit to get the cover crop in early.
Once the lettuce is out, the oats will take over the bed, crowding weeds and sucking up excess nutrients that would have been lost. Oats will continue growing into fairly cold weather, but will reliably be killed by our winters (once we hit -10° C), which makes incorporating the residue easy in the spring. Even after they have been killed, their fibrous roots will help prevent erosion.