Limp mode is a protective system, in which the vehicle limits the speed or rpm when it senses certain fault codes. Some vehicles also limit which gears can be selected, but the Spyder doesn't. Many feel that the Spyder limp mode (especially the RT) is overly sensitive, and is triggered in less than crucial situations. I tend to agree...although I feel it has a purpose for some types of failures. BRP has recently released an update to the computer(s) that makes limp mode less drastic. If you get limp mode, you will know it. The display will show an orange warning, and say "Limp Home Mode". Your rpm will be limited to 3,500 or so...no matter what gear you are in. This used to happen suddenly, not sure if new update changes that.
I would not fear limp mode, just because it shows up here a lot. It is a giant pain in the butt, especially if it comes on at speed (common with the old firmware versions), but many riders have never seen it, and others have seen it sparingly. The Internet tends to magnify everything. Remember, if it does happen, it is trying to tell you something serious. It may hit you in the head with a hammer to get your attention, but the message is important (for the most part). It is usually triggered to warn of a failure of a critical system, like the ABS or VSS, or imminent chance of engine failure, like low oil pressure. If it wasn't so easily triggered, giving false alarms, people probably wouldn't complain, but sometimes it seems like the boy that cried "Wolf!"