In late 2016, ActiveCampaign made a change to the list of “special characters” that could be used for tags.

Whilst the intention was well meant, it sent a small ripple of chaos into the ActiveCampaign universe and “the fur flew” from users whose tagging taxonomy was affected. To their credit, ActiveCampaign quickly reversed the changes and everything settled back down.

Why?
ActiveCampaign saw the possibility of some malicious code that might run if access to the “/” character is made. So, they made a decision to prevent users for using this character (and others) inside new and possibly existing tags. This meant that if you had a taxonomy like say “cat/food” and “cat/baskets” these tags might not be available to use in the future.

Currently safe special characters:
a colon: “:”
a hypen: “-“
an underscore: “_”
an ampersand: “&”

Ones to be avoided (as possibly will be “banned” in the future)
pointy arrows: < >
curly brackets: { }
square brackets: [ ]
leaning forward command line: /
leaning back command line: \

Unsure:
a “pipe”: |

The Future:
ActiveCampaign might ban some of these special characters in the future, so, to be safe, I recommend use just use the characters marked on the above “safe” list!