Digging out some brick Plastikard for a project a couple of weeks ago, I noticed it didn't behave as normal. Cutting it wasn't smooth, and when bent, it shattered. Horrible stuff to try and use. Fortunatly, I managed to cobble to gether enough good stuff for the model, but it was tight.
At the G1 birthday event, I was pleased to see Hobby Holidays, and proceeded to re-stock my supplies. Explaining the problem to the proprieter, I bent a sheet, and it shattered in the same way! Fortunatly, he accepted this was faulty and I bought a pile of normal sheets.
This isn't a problem I've enounterted before - and I've used a lot of Platikard over the years. My guess is a rare bad batch, it wasn't old, or suffered exposure to sunlight. Hopefully, it's all fine now, but do check when buying.