Are you trying convert float to numeric and you get "Arithmetic overflow error converting float to data type numeric" error? This could happen even if all was working before and now it fails. The reason should be in precision and scales settings.
When you alter view or stored procedure and you realize, previous version wasn't saved and you need it. Or you want recover SQL script immediately after management studio crash and you were lucky, that you run it at least once. There is a nice tricky way how do to it by SQL script without any 3rd party enhancement.
Sometimes it can happen, that you compare two strings, which looks exactly sames, but they are processed as not equal. Also when you use SELECT DISTINCT and still there are rows that looks same, like DISTINCT is not working. Dimensional attribute in OLAP can return errors for duplicities even you used DISTINCT for dimension. This is the right time to search invisible/hidden characters in string. RTrim and LTrim function not stripping whitespace.
If you need calculate week number from date, you can use DATEPART with parameter ISO_WEEK. Unfortunately there is nothing like ISO YEAR parameter. In case you need to show YearWeek value in some report you have to take care about correct value manually.