Connecting Problems
Afaik this game will not work online if you have no or a bad serialkey. If - for any reason - the game says you need another serial in the Options menu, read the 1.4 info.
If you see no servers at all even if you have selected the right Internet Connection above the Serverlist, apply the right Master Patch for your version of RtCW, or follow the n00b instructions.

PunkBuster Problems - EnableFIX NEW! (Succeeded after trying several other possible solutions on version 1.41; source)
The anti cheat software (that for 1.3 versions and above automatically) is part of the game, needs to be enabled for some servers. But to have PunkBuster be enabled still gave me headache in 2012. For PunkBuster to work you will need to have a good serial set in the Options of the game. If you doubt the one you're using, check out the info for RtCW 1.4 on this site. That aside..
If you see no PunkBuster button above your serverlist, you don't have PunkBuster at all. Read on the bottom of that same page exactly what version you use and find out about updates here.
But if you dó have a good serial and also the PunkBuster button is there, but it just simply cannot be set to Enabled, heres the fix: Delete the folder /pb in the RtCW folder (Program Files/Return to Castle Wolfenstein/ ). Now extract the folder /pb from this spot to the RtCW folder. After that, download and start pbsvc.exe.
(For some Windows versions it may be needed to run the file pbsvc.exe as an Administrator (right-click,properties) and if so another file called WolfMP.exe also needs to be run as Adminstrator.)

The tool HTTP-mapdownloader (named httpInjector) causes RtCW to crash because of PunkBuster thinking it's a cheat, so delete it and use the tool Map Downloader as great alternative for 1.3 and higher!

1.0/1.1 client joining 1.3 server
If a player with version 1.0/1.1 tries to connect to a 1.3 server (which are also visible in the same serverlist), the game might quit. Use button serverinfo to find out whether a server matches your own install.

1.0-1.3 map fixes
Updates to some maps (afaik 1.0-1.3): Xlabs & Boss and Norway..

JoiningTeam Problems
First of all, the most simple tiny problem might be that the default nick in the game is often refused by gameservers, so you will not be able to connect to servers, or get kicked within a few minutes. Change if needed.

The next is what always happens to people I introduce the game to: At a certain point they enter Limbo menu with key L to choose team, med/lieut whatever. After that a button with the name 'Close' (yeah, read 'Play') must be clicked. Then it takes some while to find and remember the arrows next to the 'Close' button to navigate. Anyway.. After closing this Limbo menu the spectator view returns. And than.. Nothing happens.. Still you are in Spec view. And nothing happens.. If you try again, same thing (I admit, hated the game because of this.. Felt as a bug.) It will have one of the next reasons:

• In most servers you can only join Allied / Axis in case the team has equal or less players than it's enemy. So, you join a server, in the Limbo choose Spectator and press F4 to see all players on the score list. Now return to Limbo and choose a logical team. Sometimes a clear message pops up that you have to wait a few secs. Not a bug, just a server setting to have even teams.
• Also, it might be good to know that in DM-servers it's not possible to join a team during battle. RtCW returns a message about you joining the wrong team but it's not clear at all. So wait and join the right during the Warmup between the games. Neither a bug but may be experienced as such.
• Last and certainly not the least, a bug of RtCW I have always experienced: If you know all I wrote above and you still cannot join, probably you get a small message saying you will join a team but you just don't. In this case again go back to Limbo, use the small key left of the Close button to re-enter Spectatormode. Than check what team to join and again go to Limbo and join the team.

Experienced Radeon HD 3850..4870 (oc) RtCW 1.0 Bug (8/2011)
RtCW 1.0 crashes when viewing maps MP_Dam, Boss2 and MP_Castle when using ATI Radeon 3870OC. In console "set r_primitives 2" or setting OpenGL extentions OFF via menu could have done the trick (more). But it hasn't. No working solution found yet. Simular problems occur with the HD4850. Info on this severe bug is hard to find but some can be found here and here. And I suspect the blue screens (bsods) with the HD3870 are with not-XP-systems. A new videocard B) was my solution (and it doesn't have to be an expensive one, remember it's a 2001 game..).