Well, I found a strange bug I guess. Here's a step-by-step guide to reproduce it:

• place a wooden preasure plate and and a wooden trapdoor behind it on a solid platform
• (remove the block beneath the trapdoor)
• start sneaking in a straight line towards the trapdoor while sneaking over the preasure plate that powers the trapdoor
• when you continue sneaking on the closing trapdoor you should get 0.5-1 heart damage

 I'll add a screenshot of the setup later