How to fix a score that contains corruptions

1. If you're using MuseScore 4, start with How to fix MuseScore 4 corrupted files and if still needed continue here after that.

2. Click "Show Details." You will see something like this: "Measure 30 Staff 2 incomplete. Expected: 4/4; Found: 15/16" (If you are attempting to upload a score to MuseScore.com, this message is displayed on the upload page.)
   What this means: One or more measures are corrupted—for example, a measure's duration is supposed to equal four quarter notes, but somehow it contains the equivalent of fifteen sixteenth notes, and with this much it's full. Don't panic; it's easy to fix.

3. Copy the information in the dialog to a text editor, and click "Ignore" to continue and load the score.
4. If the score has parts, delete those (File > Parts > Delete), else any fixed corruption might come back from them. Also see below, corruptions that exist in parts only do not get reported.

5. Go to each of the measures affected and count down the staves, if necessary. The measure number from the error message may not necessarily match the number shown in the score, due to pickup measures, section breaks, or modifications to the measure number offset in Measure Properties. Use the shortcut Ctrl+F (Cmd+F on Mac) and enter the number from the error message to go to the correct measure.
   Usually you can tell just by looking at it that the note values just don't add up. Now there are several methods to fix this:

   • Simply correct the note (or rest) lengths. MuseScore is clever enough to recognize "Oh, that makes sense" and fix the measure.
   • Another method is to select a corrupt measure, exchange voice 1 with any other voice and back again (Edit→Voices), and then if necessary delete a newly added full measure rest in that other voice.
   • If a measure is so badly corrupted as to be completely empty (reported as "... Found: 0/1"), there will be no notes or rests to edit in it, and nothing to select to exchange voices. In this case, it is possible to overwrite the empty corrupted measures by copying and pasting a passage over them. The measures copied must be non-corrupted, and the range of measures copied must be at least one measure greater than the range of corrupted measures so that they can be pasted starting at the measure before the corruption. Following that, you can delete the content pasted, and the measures will be correctly filled with rests.
     Another option is to select the surrounding measures, i.e. click the measure before, Shift+click the measure after, and then perform the 'swap voices' method from above
   • Another, simpler method in the above case of completely empty measures is to select everything (Ctrl+A or Cmd+A on Mac) or a range of measures containing the corrupted one (see above) and then use the shortcut for "Full measure rest" (Ctrl+Shift+Del or Cmd+Shift+Del on Mac) to fill all measures without any notes with full measure rests. There is a small downside to this: if you have measures containing only rests, e.g. a quarter rest in a 1/4 pickup measure, these also would get replaced by a single full measure rests.
   • If MuseScore crashes while trying to select the bad measure, try instead selecting the measure immediately before or after the defective measure and then use Shift+→ or Shift+← as appropriate to expand the selection to include the defective measure. Once the measures are selected, you can use Ctrl+Del (Cmd+Del on Mac) to delete both measures and then follow the procedure to insert two new measures and input the notation information to correctly recreate the two measures deleted.
   • Sometimes it may help to export the corrupt score as MusicXML, then import it back.
6. If your score had parts (see above), regenerate them.
7. Save your file. It is now no longer corrupt.

Sometimes it may happen that MuseScore crashes while you're working on the score trying to fix it, this might be due to autosave, which by default happens every 2 minutes. Disable autosave for the time you need to do the fixed and until you can save the score without it crashing.
Another possible problem is that MuseScore doesn't report corruptions in parts (MuseScore 4.0.2 and later does), but still crashes on save.
To fix that, delete the parts (Mu2 and 3: File > Parts > Delete) and generate them afresh (in the same dialog)

There's also the Repair Assistant plugin that attempts to repair some common score corruptions in the selected range.