Hidden fees are anything added on top of the sticker price per kWh: a flat connection fee just for plugging in, per-minute billing once you pass a time threshold, and idle or parking fees if you don't move your car in time. None of them show up in the headline rate, and together they can change what you actually pay more than the rate itself does.
The connection fee
Some networks charge a flat fee just for starting a session, regardless of how much energy you take. That fee behaves like a tax that gets lighter the more you charge, and heavier the less you charge.
Take a €1 connection fee on a 5 kWh top-up: spread over those 5 kWh, it adds €0.20 to your effective price per kWh. Take the same €1 fee on a 40 kWh charge, and it only adds €0.025 per kWh. The fee is fixed, but its impact per kWh isn't. Small top-ups are where connection fees hurt the most.
Per-minute billing past a threshold
Some tariffs bill by the minute instead of, or in addition to, the kWh, especially past a set time on the charger. The logic is that a car charging slowly ties up a bay other drivers could be using, so the network prices the time, not just the electricity.
Run the numbers on a 20-minute stop at 50 kW: you take on roughly 16 kWh in that window. A €0.50 connection fee on that session spreads to about 3 cents per kWh, on top of whatever the per-kWh or per-minute rate already charges you. The faster your car actually charges, the less any time-based fee costs you per kWh, because you're pulling in more energy for the same minutes on the clock.
Idle and parking fees
Once your battery stops accepting a fast charge, or once it's simply full, some networks start billing you for occupying the bay, sometimes after a grace period of a few minutes. This one has nothing to do with energy at all: it's a fee for the car, not the battery. If a network's idle fee is €0.10 a minute after a 10-minute grace, sitting for 20 extra minutes past that grace adds €2 to a session that might have cost only a few euros in electricity.
AC and DC aren't priced the same
Slow AC charging and fast DC charging usually sit on separate tariffs, because the equipment and the electricity itself cost the operator differently. A rate that looks cheap on an AC charger doesn't tell you anything about what the same network charges on its DC chargers, and the reverse is just as true.
Reading your own real rate
To find out what a session actually cost you per kWh, add every fee, connection, time-based, idle, to the energy cost, then divide the total by the kWh you actually took on. That number, not the advertised rate, is what you should compare between networks. goodcharge lists every one of these fees next to the per-kWh rate on a station's page, so the numbers you need for that arithmetic are already in front of you.