Gains goblins. Those things in your life that rob you of what otherwise would be your hard-earned gains on strength, muscle size, endurance or whatever other thing you want to develop by training.
It can be anything: a person (like that gym bro that maybe distracts you more than it's good for you), a medical condition, a commitment, or what is usually most common (and be honest here, because for most people it is one in this category), a bad habit of yours, amongst others.
Feel free to share.
Infrastructure. My main exercise is cycling to and from work because I don't like exercise, but the cycling infrastructure in my city is shite (some of the best in Britain, which says a lot about how bad it is elsewhere). Potholes, cycle paths that suddenly merge into a dual carriageway, and a complete lack of the maintenance roads get, like sweeping and gritting.
So last winter I got wiped out by black ice and hurt my leg, stopping me from cycling for a few days, and last week I got wiped out by rotten leaves from last autumn, which wrecked my derailleur and has put the bike in the shop for a week. Doesn't just steal my gains, but either my money getting taxis home from work or my time by waiting up to an hour for a bus.
My condolences.