#2. Vortigaunt stomped headcrab. Developers even made a variant of meeting vortigaunt and headcrab closely, despite of fact that there are no such situations in game. So they did not notice bug which spawn corpse of usual headcrab in case of stomping on poison headcrab. 
#3. (secret). 
#4. Bag push into head. If NPC is moving by scenario from A point to B point, and we slaps his head by bag or something, then model of bag partially crosses with NPC's model. Consequently, game is thinking that NPC got stuck and failure occurs. Eventually, NPC will be moved to B point forcibly. 
#5. Super-strength of guard. Objective of this NPC is strongly pushing player's model at the time of blowing. By this, developers wanted to show that taser's blow is enough strong and don't want to mess with these guys. But there is one problem as ground friction. Developers attributed to this taser very strong impulse for enough showing of blow effect, if player is standing. But there is almost no friction in air, so if you locates that blow will be from below, you will strongly take off.

#6. Kill G-Man. Unlike HL-1, here G-Man has limited health. He appears in that point when we starts survey from other side. Also if we get missile system, gunship will appear. Gman is enemy for gunship default. So if we firstly use survey device after getting missile system, then Gman will be dead and further you can find his corpse.
#7. Pistol mega-fire. While pressed LMB, shots occurs through same timings. If you'll press RMB more, then shots will be stopped (as shown), but system will continue to count shots (remaining in clip) through these same timings. At the time of releasing RMB, all becomes to normally working and system makes all shots missed simultaneously. 
##8-20. (secrets). 
#21. Create flying missile. Game allows to create flying missile by command "npc_create rpg_missile". For creating a lot of missiles by one key, for example T, you should type these strings: 
alias a "npc_create rpg_missile"  (Create new command "a")
bind t "a;a;a;a;a;a;a;a;a;a"    (Binding Т key to multiple executing of 'a' command)
In this case, we makes 10 missiles at one time.
#22. Strafejump. Similar to CS 1.6, while pressing forward, side move keys and moving mouse to same side, we're increasing speed of run enough. Trick named as "Prestrafe". If we jumps in so doing, length of jump will be much more and it's named "strafe jump". Also, if we press acceleration of run, length of our jump will be so big that we easily jump through this impediment. 
#23. (secret). 
#24. Barrel explodes from touch. Developers wanted to show big strength of gravi-gun by feature that while throwing explosive object by gravi-gun, object will explode by force of blow. They attributed just explode from first touch after throwing. But they didn't take to account that player can throw it vertically and after that catch it by same gravi-gun.
#25. Bunnyhop. As in CS 1.6, this trick allows to keep speed after landing. But here you do not need even strafes. We get acceleration by forward move key pressed. There are needed strings for executing bug:
  bind mwheeldown +jump   (Binding jump command to moving of mouse wheel down, you should quickly move wheel down at the time of every landing) or script such that:
  bind mouse2 ";+jump;wait;-jump;wait;+jump;wait;-jump;wait;+jump;wait;-jump;wait;+jump;wait;-jump;wait;+jump;wait;-jump;wait;+jump;wait;-jump"
  fps_max 30      (This script allows to change of wheel turning to single pressing key, in this case - RMB, also limiting of Frames per second [FPS] make it easier.)  

#26. Elevator bunnyhop from object. Based on #25. This bug is using script from bug 25. If we take object and push it under legs, we can jump on it. After jumping game quickly blocks taking and it stays on ground. But if we'll frequently use script then we'll start jumps on it. We'll continuously make jumps on object, which continuously approaching us by taking. Eventually, we can gain unlimited height and by moving object to side for little we'll move with horizontal directions.  
#27. (secret). 
#28. Landing bug. At the time of flying, if we touch the edge of barrel, then it will take almost all our falling impulse. We'll loose speed almost without injuring. Barrel will quickly take off into side. 
#29. Walking in Test chamber from HL-1. In beginning of game at the time of G-Man's monolog we're see test chamber from HL-1 and other places. Map has already loaded and no more loading. So, all these places are hidden on map somewhere. By cheat-mode you can get it. To actuate this mode you need type this:
  sv_cheats 1
  noclip
#30. How to kill Breen. Based on #29. One of these hidden places is Breen's room. We can kill him there by command for example "impulse 101". After that, Breen's image at beginning will disappear. 
#31. Shortcut through gates. Based on #22. Using strafe jump, we can jump around closed gates and continue the adventure, but without car. 
#32. (secret). 
#33. (secret). 
#34. Shortcut by button through ceiling. Game is protected from actuating buttons through walls, unlike HL-1. But this one locates closely to floor. If we jump to ceiling below, we'll appear in ceiling texture for one moment. If we'll catch this moment and press 'use', then button will be activated.
#35. (secret). 
#36. A lot of vortigaunt's corpses. So, this NPC is not intended to shooting him from upgraded gravi-gun.
#37. Computer is deadly dangerous. While locating on small movable object like comp, we have unstable state. In so doing, if we'll be too close to that combine's wall, then, sooner or later, we'll be too snuggled to it. These walls have a feature that any pushing by this wall will be deadly. In this case, game will think that wall stomped us. 
#38. (secret). 
#39. Dude without eyes. Based on #4. We shouldn't see him so close. So he's not designed completely. By scenario, he's moving between two points located some above floor. There are no things that can prevent it. You can get this place only by Noclip mode as in bug 29. 
#40. Pushing barrel in gates. Pinned object in gates is in gate's textures. In so doing, gates strongly tries to push it out. During time, gates will make it, and if player will be closely, that he can be pushed or crashed.
#41. Pushing barrel in door. Similar to #40.
#42. (secret). 
#43. Shortcut through 2nd gates by boat bug. Trigger of gate closing is a thin plane which locates on edge line of tunnel. It works at the time of player's crossing. But if you'll push the boat forward from yourself, and from maximum distance actuate it, you'll not go through way of this distance between your location and boat middle point, but you programly will be teleported to this point. If the trigger's plane will be between these points, then, eventually, gates will not be closed until you pass it.