OSCam has a reletively new feature called ecmwhitelist. What this feature does is allow a user to define valid ECM lengths a particula card, therefore blocking anything unusual in length. A lot of people see this feature as a way to stop providers blocking cards, as it prevents anything unusual reaching it.

This whitelist goes in oscam.server in the appopriate reader entry for your local card. The first part defines the CAID, then after the @ you have your ident, and then after the colon (:) you have your allowed ECM hex lengths. If your card has multiple CAID/Idents, then just add another entry, making sure each entry is separated by a semicolon (;)
Code:
ecmwhitelist = 0500@042400:52,57
The above is the entry I have for my JSC Sports card, which is being read in OSCam. Not too long ago, it was a bit different as below;
Code:
ecmwhitelist = 0500@042400:2F,2A
I found today that my card wasn't clearing channels. I eventually discovered this was why.

Problem. Sometimes, the length of the ECM can change. The provider will sometimes tweak things. You want to save your card from being murdered, but the provider has changed something, and you can't watch your channels anymore.

Solution. Quite simple really, you need to know where you get the information from to obtain the hex code that describes the length of the ECM. If you are running OSCam Web Interface, then all you need to do is look at a line of information. This is from JSC Sports +8 The following information will be in your log files, but I prefer to read it straight from the status page of the Web Interface.

Code:
2012/02/04 23:38:12 ABC123 c jsc (0500&042400/20FC/52:9827): found (303 ms) by upper_slot
The above tells me that the hex length of the ECM is 52 (as highlighted in red). I now know that I can add that to the ecmwhitelist. I also discovered a length of 57, so I added both, and so far that is working.

Knowing this allows you to build your own lists. I am still not so sure about these supposed killer ECMs or EMMs, but I do know that it is good to be restrictive, but being restrictive sometimes blocks things out, so if you ever see a channel not clearing on your card that should be, as well as your services, check the ECM whitelist against the ECM on your OSCam log, because that could be what has changed!