Pk_campaign parameter mangling

Hi!

After sending a newsletter to our customers we found something strange: a few users clicking on the links had the pk_parameters mangled, eg. “pk_campaign=newsletter” became “pk_campaign=afjfyfggfe”
These same users seem to come from different countries. As most of our customers are in Germany I suspect their browser plays some tricks.

Just out of curiosity: has anyone seen this? Do you know, which broser or extension might be doing this?

Thanks!