Q:

Importance of concurrency issues in web applications

With respect to this question, is it always necessary to have perfectly concurrent-safe web-application or can a developer afford to let some possible concurrent issues untreated (e.g. when money is not involved) because probability of them happening is very low anyway. What is the best practice?
NOTE: If I say concurrent issues I mean the issues raising from overlapping executions of scripts. I do not mean the multi-user issues like classic lost-update with the timestamp solution cause probability of these things is imho significant and I am pretty sure here that the best practice is to always treat them.

A:

If you're going to run your code on a web server, you should always write your code in such a way that multiple copies of it can run at the same time.  
It's really not that difficult.  In most cases database transactions around state modifying operations is all that is needed (or using a lock file, or any of the other well-known solutions to this problem).
Note: if all you're doing is reading data then there are no concurrency issues.

