I like to distinguish three different types of conflict from a version control system (VCS): textual syntactic semantic A textual conflict is one that is detected by the merge or update process. This is flagged by the system. A commit of the result is not permitted by the VCS until the conflict is resolved. A syntactic conflict is not flagged by the VCS, but the result will not compile. Therefore this should also be picked up by even a slightly careful programmer. (A simple example might be a variable rename by Left and some added lines using that variable by Right. The merge will probably have an unresolved symbol. Alternatively, this might introduce a semantic conflict by variable hiding.) Finally, a semantic conflict is not flagged by the VCS, the result compiles, but the code may have problems running. In mild cases, incorrect results are produced. In severe cases, a crash could be introduced. Even these should be detected before commit by a very careful programmer, through either code review or unit testing. My example of a semantic conflict uses SVN (Subversion) and C++, but those choices are not really relevant to the essence of the question. The base code is: int i = 0; int odds = 0; while (i < 10) { if ((i & 1) != 0) { odds *= 10; odds += i; } // next ++ i; } assert (odds == 13579) The Left (L) and Right (R) changes are as follows. Left's 'optimisation' (changing the values the loop variable takes): int i = 1; // L int odds = 0; while (i < 10) { if ((i & 1) != 0) { odds *= 10; odds += i; } // next i += 2; // L } assert (odds == 13579) Right's 'optimisation' (changing how the loop variable is used): int i = 0; int odds = 0; while (i < 5) // R { odds *= 10; odds += 2 * i + 1; // R // next ++ i; } assert (odds == 13579) This is the result of a merge or update, and is not detected by SVN (which is correct behaviour for the VCS). int i = 1; // L int odds = 0; while (i < 5) // R { odds *= 10; odds += 2 * i + 1; // R // next i += 2; // L } assert (odds == 13579) The assert fails because odds is 37. So my question is as follows. Is there a simpler example than this? Is there a simple example where the compiled executable has a new crash? As a secondary question, are there cases of this that you have encountered in real code? Again, simple examples are especially welcome.