On the way to my game last Sunday, I had a random thought. If an ineligible player is the first to touch a forward pass, it's a five yard penalty. However, if an originally eligible receiver makes themselves ineligible by stepping out of bounds and then touches the forward pass, it's a loss of down.
"Why are the penalties different?", I asked myself. Then it dawned on me - the first foul (the ineligible player touching the pass) nearly always occurs because someone somewhere has messed the play up (for example, a receiver lining up on the line when he should be off it, or the QB coming under pressure and throwing the ball anywhere he can) - it's a technical foul, just like a false start or an illegal motion. On the other hand, the second foul could occur due to the receiver deliberately going out of bounds to avoid a block, and I assume this is what happened before the current rule was in place - it's more of a dishonest foul hence the slightly stricter penalty.