Context-Sensitive Help
Matlab has a hidden/unsupported built-in mechanism for easy implementation of context-sensitive help
Pure Matlab
Legend '-DynamicLegend' semi-documented feature
The built-in Matlab legend function has a very useful semi-documented feature for automatic dynamic update, which is explained here.
Spicing up Matlab uicontrol tooltips
Matlab uicontrol tooltips can be spiced-up using HTML and CSS, including fonts, colors, tables and images
tic / toc – undocumented option
Matlab's built-in tic/toc functions have an undocumented option enabling multiple nested clockings
Spy Easter egg
The built-in Matlab function spy has an undocumented feature (Easter egg) when it is called with no input arguments.
ismembc – undocumented helper function
Matlab has several undocumented internal helper functions that can be useful on their own in some cases. This post presents the ismembc function.
Undocumented profiler options
The Matlab profiler has some undocumented options that facilitate debugging memory bottlenecks and JIT (Just-In-Time Java compilation) problems.