According to http://www.wowwiki.com/API_GameTooltip_AddLine there isn't a limit to the number of lines you may add to the tooltip, but I believe there is a limit. I have an addon that adds various amount of lines to a tooltip and when putting 210 lines into the tooltip, wow instantly crashes.
Well, while I do agree with your argument, artificially limiting the number of tooltip lines does avoid the crash. But my manually attempt isn't identical to the way the addon does it and the difference is basically that I manually opened the tooltip from the command line (using a macro).
Anyway, I have successfully avoided any crash since I imposed the limit to the tooltip.
I have now tried to isolate the bug only to have the bug vanish on me. I am now running my original addon (the one that triggered the crash) and I cannot reproduce the crash. I can only consider some wow API or its Lua interpreter to malfunction in some exotic way. :-/
I think I am referencing invalid item:* links (as they are provided by LibPeriodicTable) and asking wow to fetch the tooltip. But I thought fetching invalid items wouldn't crash wow. Worst I'd expect to be disconnected...
But the client crash occurs after my code has enumerated all the links (extracted name and quality info) and wow often manages to display the tooltip before actually crashing. This strengthens my suspicion that something is corrupting wow's memory, causing a delayed crash. I can only hope Blizzard puts in better API parameter verification... :-P
For now I have worked around it my using GetItemInfo directly instead of using a mix of GetItemInfo(itemlink) and tooltip:SetHyperlink(itemlink) to get item and enchant information (as this can crash wow).