Mozilla is warning users that when Firefox — and Google’s Chrome — reach version 100, major websites may no longer identify them properly, and not work properly as a result.

Firefox is currently on version 97, while Chrome is on version 98. Once those are updated to version numbers with three digits, Mozilla says there are could be inconsistent problems across an unpredictable range of websites.

According to Mozilla, website servers examine what’s called the User-Agent in order to determine which browser is being used. They then use that information to configure sites so that they display correctly.

Mozilla points out that sites had to cope with a similar issue arose with the move from single- to double-digit version numbers, “so hitting the three-digit milestone is expected to cause fewer problems.”

Nonetheless, Firefox and Chrome developers are running experiments, and also logging issues. Currently, the list of sites reporting bugs with a version 100 include T-Mobile, Yahoo, and Daimler.

Similarly, the developers working on Google Chrome have a “backup plan to use a flag to freeze the major version at 99.”