Guild Wars 2

Gargantula’s life resets to full when at 50-25%

Here, take a look at the wiki page. Possible it is a bug, but probably not. Probably just missing an NPC speech to indicate what is happening.

http://wiki.guildwars2.com/wiki/Gargantula

Essentially, you have to kill Gargantula 3 times; each time you take her health down to a certain percentage (1st time 75%: 2nd time 50%: 3rd time 25%) she will regain all of her health, summon many spiderlings and become invulnerable to damage as long as any spiderlings are alive. Once all of the spiderlings are defeated, Gargantula becomes vulnerable to damage again, and with seemingly less armor each time. The third time you will actually be able to defeat her and gain the rewards.