This is a bit like the ladder problem but involves motion in two perpendicular directions.
The following version is quoted from this Quora answer by Peter-Ripota (who unfortunately seems to think it represents a real contradiction in Special Relativity)
<<<start quote
Look: This is the Einstein-train at rest before a garden-fence:
As you can see, a ball fits through the gaps of the (stationary) fence. Now, Let’s start the train, giving it so much boost that it will reach nearly c, the velocity of light. Remember, we’re still in an inertial frame, and it’s a thought experiment devised by Einstein himself! What now happens, depend on where you stand and watch.
Case 1: You as an observer are resting relative to the garden-fence:
According to Einstein’s length-contraction, train plus ball contract (relativistic effects in red), the ball can be easily thrown through the gaps of the fence.
Case 2: Now you, the observer, are standing on the train (relativistic effects in red):
In this case, the rest of the world moves along your view-point with nearly c, which means: The world will contract before your eyes. This is all according to the relativity-principle which sates: “moving” or “resting” is always relative, never absolute. You need two objects to determine a velocity. And we are always regarding inertial frames – the train is moving in a straight line with constant velocity.
But now you would be unable to throw the ball through the gaps – they are too small!
>>>end quote
Or are they?
Here’s the thing.
In order to get the contracted ball through the fence it must be moving in the perpendicular direction as well as along the tracks beside it. But if the front (wrt train motion) and back of the ball are started simultaneously in the fence’s frame of reference, then in the train’s frame the front is started earlier. This distorts the ball into an ellipsoid whose minor axis is small enough for it to slip diagonally through the gap.
Don’t believe me? Well the proof is a high school geometry exercise and ability to do it for yourself should be a prerequisite for making public comments about relativity.