From time to time, either you or one of your opponents will experience server or PC problems during a hand, and will be unable to act in turn due to a lag, disconnection or freeze. Depending on the site in question, that player may be put automatically all-in at that point for the chips they have already committed to the pot, able to win the main pot, but excluded from any side pot that may be created. The idea is to protect players from losing their investment in a pot just because of connectivity problems - sometimes such disconnections will work in favour of the player and sometimes against. Of course, if you suspect that someone is abusing the all-in protection rule, deliberately disconnecting themselves in a difficult situation to avoid having to call a bet, you should report them to the support team.
Many sites do not allow automatic all-ins for pot-limit and no-limit games - it is important to check the specific rules for that site if you intend to participate in such games. In fact, in the past couple of years many of the major sites have removed the automatic all-in facility even for limit games. Even on those sites where automatic all-ins are still allowed, players are only allowed a given quota of all-ins (typically one or two) within a 24-hour period; once that quota has expired any further disconnections are likely to result in a loss of the pot by default. However, most sites do have an all-in reset procedure whereby players can request that their all-ins be restored. If your all-in quota expires, it is important to request a reset straightaway, since otherwise you may end up needlessly forfeiting a big pot if your prevailing connection problems persist.