Our current GM used to love to mix old and new characters across campaigns. A couple books to his head later and he doesn't do that anymore.
Things that get PCs enemies...
- No longer allowed to mail Lofwyr a "Tickle Me Lofwyr" doll.
- No longer allowed to mail Damien Knight a video of his goons getting killed.
- No longer allowed to send an accompanying MST3K Riff track with the said video.
- No longer allowed to mail your former Johnson a live grenade.
- No longer allowed to mail death threats to the governor of Seattle.
- Nor allowed to mail undeath threats to the governor.
- You are hereby banned from using the US Postal Service for anything! Period!