Long time snorkeler/freediver/water dog here. I've never had problems taking mask off underwater/clearing before I did my OW. I went through the first 3 dives/several internations of all the mask clearing just fine. On the last and final dive where we had to run through all our skills, things got dicy for me. The mask I was provided for class broke on me the day before the last dive. I was given a low profile style mask as a replacement. It was the first time I had worn the mask. When time came for me to do the mask clearing 3(full removal) I'm thinking alright no biggie lets do this, been doing this all weekend. When I started filling the mask/pulling it off, there had been a seal created around my nose, only my nose, not eyes. When I pulled the mask away from my face the seal around my nose broke, the negative pressure immediately flooded my nose with water. Mentally I knew exactly what happened, I knew I could breath, in my head I was telling myself to stay calm but my body was not hearing that. Body was in full blown panic. It took several minutes for me to calm down. After that I was almost terrified to try mask clearing again even though I had done it a million times before. I couldn't even get in the water without a mask and swim without my body thinking I was going to drown. It took me getting into my pool and forcing myself to go underwater no mask, with a snorkel for hours before I felt comfortable again. It was one of the strangest thing that's ever happened to me. Nowadays it's no biggie, I take my mask off all the time while diving to practice mask clearing.