In <i>increasing</i> order of importance: 1. Interruptions and context-switching 2. WTFs that are now everywhere, in language syntax, in libraries, etc. 3.

In <i>increasing</i> order of importance: 1. Interruptions and context-switching 2. WTFs that are now everywhere, in language syntax, in libraries, etc. 3. Incomplete or inconsistent libraries, with unexpectedly high learning curves (e.g. due to lack of good documentation), recursive dependencies, etc. Half the time goes into just wiring things together, whether code or data.