When you confirm a segment, sometimes you can see a red exclamation mark or an orange thunder in the status column. The former indicates that the number of uninterpreted formatting tags in the target segment is not equal to the number of tags in the source segment., while the latter tells you the same about inline tags.
You can export a translation with warnings in it but you cannot export one that contains errors. Double-clicking error and warning signs shows their causes. You can also instruct memoQ in this dialog to ignore a warning, but not an error. Alternatively, you can invoke the Resolve errors and warnings tab showing a list of all warnings in the document(s) by choosing Resolve Errors and Warnings on the Review ribbon tab or from the Run QA dialog by checking the Proceed to resolve warnings after QA check box.