If you use a Tag that isnt in the website tag database, for example "populate" "if", the next page refuses to load but doesnt tell you why, at best there is a red cross under the tag list...
"did the page bug, it looks all fine?" "why is the page bugging" "shall i copy and paste everything again onto the same page and fill the tags differently" "why are my perfectly nice tags messing up agAAAIN"
2 months later i realise, it needs tags that "existed already"
It would be nice to have a big line that sais "Populate tag not in database"