Cookies can't be more than 4KiB in size
Did you know: you can't reliably store more than 4KiB (4096 bytes) of data in a single browser cookie? I didn't until this week.
I'd never before stopped to think about whether or not there was a limit to how much you can put in a cookie. Usually, cookies only store very small string values, such as a session ID, a tracking code, or a browsing preference (e.g. "tile" or "list" for search results). So, usually, there's no need to consider its size limits.
However, while working on a new side project of mine that heavily uses session storage, I discovered this limit the hard (to debug) way. Anyway, now I've got one more adage to add to my developer's phrasebook: if you're trying to store more than 4KiB in a cookie, you're doing it wrong.
Actually, according to the web site Browser Cookie Limits, the safe "lowest common denominator" maximum size to stay below is 4093 bytes. Also check out the Stack Overflow discussion, What is the maximum size of a web browser's cookie's key?, for more commentary regarding the limit.
In my case – working with Flask, which depends on Werkzeug – trying to store an oversized cookie doesn't throw any errors, it simply fails silently. I've submitted a patch to Werkzeug, to make oversized cookies raise an exception, so hopefully it will be more obvious in future when this problem occurs.
It appears that this is not an isolated issue; many web frameworks and libraries fail silently with storage of too-big cookies. It's the case with Django, where the decision was made to not fix it, for technical reasons. Same story with CodeIgniter. Seems that Ruby on Rails is well-behaved and raises exceptions. Basically, your mileage may vary: don't count on your framework of choice alerting you, if you're being a cookie monster.
Also, as several others have pointed out, trying to store too much data in cookies is a bad idea anyway, because that data travels with every HTTP request and response, so it should be as small as possible. As I learned, if you find that you're dealing with non-trivial amounts of session data, then ditch client-side storage for the app in question, and switch to server-side session data storage (preferably using something like Memcached or Redis).