Playtesting during game jams is tough.
You play the game a lot, naturally, you check the balance, catch bugs and most importantly assess if it's fun. But, your mind is spoiled - you know the exact game rules - you've gamedesigned and programmed everything after all. But players won't magically know those rules! If you have complex mechanics that seem obvious to you, they probably won't be apparent to players.
We've made this mistake again this time, and a really bad one at that. Our game is a sea trading simulator (you can play it here) where you need to stockpile food to sustain your crew during lengthy voyages. We created several food types, some of which last longer and are more effective - for example 10 Meat will last longer than 10 Hardtacks. But, we forgot that the player starts with only Hardtacks and it's not clear that other options exist! To make matters worse one of the food types is Rum, and you certainly don't expect the crew to stave off hunger by merely drinking Rum.
So what happens is - there is a food rotation and sometimes there is no Hardtacks available to buy on the map - only Rum. And you'll just die if you don't know this secret! Since everyone on the team already knows it - the problem goes completely under the radar - especially when you have so much to do - polishing, balancing, and fixing bugs.
This is the lesson we learn almost every jam - to brainstorm the question "Are all of the game rules clear to the first-time player?". How do you tackle this problem? :)