Just so as you know, the USCIS Form I-751 is filed to remove conditions from a conditional green card so that you can get a regular alien registration card. A lot of readers write asking what will happen if they are unable to file the paperwork on time because they do not have to money to pay the fees or forgot about it or something else went wrong. The simple answer is that this is something too important to ignore. Actually, if you are an immigrant in this country, you should allocate funds for all the future paperwork ahead of time and that must include not just the fees and the expenses related to making photo copies or photographs or mailing but also attorney fees. These funds should be set aside in a bank account and should not be touched till the day you take the oath as a naturalized citizen. I would recommend cutting back on going out, clothes, vacations, etc. to save for immigration related expenses. The USCIS grants fee waivers only in exceptional circumstances (American people do not like having to pay for immigrants to come and live here at their expense) and it is a huge hassle to do the paperwork for getting an exemption.
If you do not file the paperwork to remove conditions on time, you will basically be out of status. The USCIS will assume that you have abandoned your permanent resident status and are not interested in pursuing it. Since you will be in no legal status, deportation proceedings can be started against you, particularly if you happen to be at wrong place at wrong time. In any case, an application not submitted on time is often delayed even more and can be rejected. Those of you who forgot about it should file it as soon as possible and those of you who do not have the money to pay the fees should seriously consider borrowing from friends and family members.