Unfortunately this issue has been hard to pin down. It seems that if the participant responds with a quotation mark (') in a long free text question on the last group of a survey, an sql error is triggered. I can reproduce this a will on my own installation (Version 1.85+) but not on the demonstration installation on limesurvey.org.

To reproduce on my machine:
1) create a study presented group by group with at least two groups.
2) in the last group, create a long free text question.
3) activate the study and run it. In the long free text question, include a quotation mark (') in your response.

This triggers the following error:

"An error occurred saving a response to survey id Demonstration of long free text error - 46588

DATA TO BE ENTERED:
46588X204X1094: 33
46588X205X1095: I'm trying to understand.

SQL CODE THAT FAILED:
UPDATE lime_survey_46588 SET submitdate = '1980-01-01 00:00:00', 46588X205X1095 = 'I'm trying to understand.' WHERE id=1

ERROR MESSAGE:
You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'm trying to understand.' WHERE id=1' at line 1"