An odd one: envelope glue

[Pamplemousse]

Here's something I'd like to ask people; have any of you encountered an envelope or Jiffy bag where the glue seemed both 'wet' and smelled strongly of vinegar? I did yesterday (it contained a book I'd ordered) and it's really bothering me. Was it something harmful? Had something contaminated it that's harmful?

It's difficult to get out of my head... :weep: