Hardest types of bugs to track? [closed]

Question

Heisenbugs:

A heisenbug (named after the Heisenberg Uncertainty Principle) is a computer bug that disappears or alters its characteristics when an attempt is made to study it.

Race conditions and deadlocks. I do a lot of multithreaded processes and that is the hardest thing to deal with.

Bugs that happen when compiled in release mode but not in debug mode.

Any bug based on timing conditions. These often come when working with inter-thread communication, an external system, reading from a network, reading from a file, or communicating with any external server or device.

Bugs that are not in your code per se, but rather in a vendor's module on which you depend. Particularly when the vendor is unresponsive and you are forced to hack a work-around. Very frustrating!

We were developing a database to hold words and definitions in another language. It turns out that this language had only recently been added to the Unicode standard and it didn't make it into SQL Server 2005 (though it was added around 2005). This had a very frustrating effect when it came to collation.

Words and definitions went in just fine, I could see everything in Management Studio. But whenever we tried to find the definition for a given word, our queries returned nothing. After a solid 8 hours of debugging, I was at the point of thinking I had lost the ability to write a simple SELECT query.

That is, until I noticed English letters matched other English letters with any amount of foreign letters thrown in. For example, EnglishWord would match E!n@gl##$ish$&Word. (With !@#$%^&* representing foreign letters).

When a collation doesn't know about a certain character, it can't sort them. If it can't sort them, it can't tell whether two string match or not (a surprise for me). So frustrating and a whole day down the drain for a stupid collation setting.

Threading bugs, especially race conditions. When you cannot stop the system (because the bug goes away), things quickly get tough.