The question_answers id are stored in the questions field defined as VARCHAR(255) that can be too small as shown in a cloze question like

{100:MULTICHOICE:%0%0~%1%1~%2%2#~%3%3~%4%4~%5%5~%6%6~%7%7~%8%8~%9%9~%10%10~%11%11~%12%12~%13%13~%14%14~%15%15~%16%16~%17%17~%18%18~%19%19~%20%20~%21%21~%22%22~%23%23~%24%24~%25%25~%26%26~%27%27~%28%28~%29%29~%30%30~%31%31~%32%32~%33%33~%34%34~%35%35~%36%36~%37%37~%38%38~%39%39~%40%40~%41%41~%42%42~%43%43~%44%44~%45%45~%46%46~%47%47~%48%48~%49%49~%50%50~%51%51~%52%52~%53%53~%54%54~%55%55~%56%56~%57%57~%58%58~%59%59~%60%60~%61%61~%62%62~%63%63~%64%64~%65%65~%66%66~%67%67~%68%68~%69%69~%70%70~%71%71~%72%72~%73%73~%74%74~%75%75~%76%76~%77%77~%78%78~%79%79~%80%80~%81%81~%82%82~%83%83~%84%84~%85%85~%86%86~%87%87~%88%88~%89%89~%90%90~%91%91~%92%92~%93%93~%94%94~%95%95~%96%96~%97%97~%98%98~%99%99~=100#}

see
http://moodle.org/mod/forum/post.php?reply=621285
To what size it should increased give the size of id in large installations?