When adding values for a multi-select field, there is a suggestion that you can use HTML tags to make the values bold or colored or whatever.
I have done this to some of my multi-select fields, and this has broken filters that were looking for them.
Previously I had values "regression" and "degradation", which I changed to "<b><font color=red>regression</font></b>" and "<b><font color=orange>degradation</font></b>". Now any filters that are looking for degradation or regression are broken.
Instead I have to change the filters to include the HTML tags. What's more, the Basic filter creator will no longer work with this field, instead you have to use JQL when doing filters.
This rather makes the ability to add HTML tags useless, as it makes it very difficult for the average user to filter on these fields now.
The suggested fix is to only parse the HTML tags when displaying the multi-select field, but to strip it out for filtering purposes.