Regression
From OdaWiki
What is regression testing
Regression testing determines at which revision a bug or feature was introduced.
How to do a regression test
Find a revision from which the bug was absent. Then find a revision in which the bug was present. Pick a revision in the middle, and test that. Now you know which suspect block of revision to test next. Divide and conquer. A bug in a block of 1024 revisions should only take about 10 tests.