There are certain things that you probably shouldn't laminate. For instance, while it is not technically illegal, the Social Security office prefers that you not laminate your Social Security card. Many employers, or others who wish to verify your identity, will not accept a laminated card because it just seems too easy to fake. This rule is true for birth certificates as well, especially because birth certificates often have an embossed seal on them. If you laminate the certificate, that seal flattens and will no longer appear official.
It's extremely inadvisable to laminate a receipt these days, since most are created using thermal paper. If you've ever left one of those receipts in your car for any length of time, you know that they eventually become unreadable when exposed to heat. Most lamination is nothing but exposure to heat, so beware.
Another thing people often like to laminate is newspaper, which deteriorates rapidly, getting yellow and eventually falling apart. You might think that lamination would preserve the paper, but that's not entirely true. Even with lamination, a newspaper will still turn yellow and fade over the years. The lamination might keep it together for longer, but it's not going to actually preserve the item. Some even advise against laminating newspaper altogether, particularly if you're a collector interested in keeping the value of that particular newspaper. A laminated newspaper clipping will have far less value than one that is not laminated.
Obviously, you're laminating something because you want to protect it and keep it safe. But that is also a reason to consider not laminating, because if you run into trouble during the process (with something as simple as air bubbles or a stray hair, for example), you might end up destroying something that's one-of-a-kind or extremely important to you.