An on-line store does not work properly (situation is possible after upgrade to 5.5, or may be independant from upgrades) - javascipts are not loaded, clicking "Add to Cart" results in the error message or nothing happens at all.
Using Firebug an error similar to this one can be seen:
SyntaxError: unterminated string literal @ http://storename.tld/index.php?StoreID=ID&act=js:7614";
There are several lines in "Incorrect Login Alert" (Home > System> Settings> Security > Login Settings), this breaks an on-line store. Issue is a bug with id #PBA-52662 ("It is possible to break an on-line store by having several lines in "Incorrect Login Alert"") .
Remove the new paragraph from the incorrect login alert and synchronize the store.