If you login to a site while in "private browsing" mode on Safari (both iOS and OSX) all JS on the site will be broken, due to toolbar setting a localStorage
setItem, which is not supported in private browsing mode.
I see three options:
1) Change the localstorage to cookies (not sure if this will have same issue, needs to be explored).
2) introduce some sort of shim to avoid all the JS from breaking, like this one: https://gist.github.com/philfreo/68ea3cd980d72383c951
2) provide message letting the user know they cannot use the site logged in with private browsing mode on. (not ideal obviously)
for reference of the issue:
[Error] QuotaExceededError: DOM Exception 22: An attempt was made to add something to storage that exceeded the quota.